Background Overweight and obesity are considerable public health issues internationally as well as in Sweden. On a global level, the obesity prevalence has nearly doubled over the last 30 years. Currently in Sweden, more than one third of all women, and slightly more than half of all men, are either overweight or obese. The long-term results of obesity treatment programs are modest as reported by other studies. The importance of extending the focus to not only obesity treatment, but also prevention of weight gain, has therefore been emphasized. Aim The overall aim of this thesis is to explore the concept of primary weight maintenance (PWM) and to increase the knowledge of the attitudes, behaviours, strategies and surrounding circumstances that are important for PWM in a Swedish middle-aged population. Material and methods All study participants were recruited based on their previous participation in a health survey in their home setting; The Västerbotten Intervention Programme (VIP) in Västerbotten Sweden (paperI-IV), or the Upstate Health and Wellness Study in Upstate New York (IV), USA. All subjects had participated twice, with a time period of ten years between health surveys. The prevalence of obesity between the years 1990-2004 was calculated for VIP participants (paper I). Ten-year non-gain (lost weight or maintained body weight within 3% of baseline weight) or weightgain (≥3%) was calculated for individuals aged 30, 40, or 50 years at baseline. A multivariate logistic regression model was built to predict weight non-gain. In-depth interviews were conducted with 23 maintainers and four slight gainers in Sweden and analysed using Grounded Theory (paper II). A questionnaire study was conducted including 2138 Swedish and 2134 US participants (paper III and IV). Analysis of variance (ANOVA), correlation, and linear regression were performed to identify attitudes, strategies, and behaviours that are predictive of PWM in different age, sex and BMI subgroups in Sweden (paper III). Further, the pattern of ten-year weightchange (% and kg) in 1999-2009 was calculated for Swedish and US women within different subgroups (paper IV). ANOVA, correlation and chi-squaretests were conducted to contrast eating and exercise habits between the two countries that may explain the differences in weight change. Results The prevalence of obesity (BMI ≥30) in Västerbotten increased from 9.4% in 1990 to 17.5% in 2004 (I). Older age, being female, being overweight at baseline, later survey year, baseline diagnosis of diabetes, and lack of snuff use increased the chances of not gaining weight. Based on the in-depth interviews, describing attitudes, behaviours and strategies of importance for PWM, a model was constructed (II). Weight maintenance was characterized as “a tightrope walk” and four strategies of significance for PWM were described as “to rely on heritage”, “to find the joy”, “to find the routine” and “to be in control”. The questionnaire study aimed at identifying predictors of PWM in different age, sex and BMI groups (III). The pattern of significant predictors was widely disparate between different subgroups. Of 166 predictors tested, 152 (91.6%) were predictive of PWM in at least one subgroup. However, only 4.6% of these were significant in half of the subgroups or more. The mean percent weight changes (in all cases weightgain), between 1999-2009 for Swedish and US women, were 4.9% (SD=5.8) and 9.1% (SD=13.7) respectively (p for t-test˂0.001) (IV). For the US women, the largest weight change occurred among the 30 year olds for all three BMI strata. For the Swedish, it was seen among overweight and obese 30 year old women. The largest difference in ten-year weight change between the two countries for any two matched subgroups was seen in normal weight 30 year olds. Significantly more of the women in this Swedish subgroup stated having more of healthy behaviours. However, there was a tendency for unhealthy behaviours to be strongly associated with greater weight gain in the US, but much less so in Sweden. Conclusion: Younger individuals, those of normal body weight, and those without health conditions (e.g. diabetes type 2) and cardiovascular riskfactors – were the least likely to maintain their weight over the 10 year period (I). Educational efforts on the prevention of overweight and obesity should therefore be broadened to include those individuals. The in-depth interview study showed great variety with regard to attitudes, strategies and behaviours important for PWM (II). The results from this study informs health personnel about the need to tailor advice related to body weight, not only to different sub-groups of individuals trying to lose weight but also to subgroups of primary weight maintainers who are trying to maintain weight. This statement was also supported by the questionnaire data, where the large disparity in the pattern of significant variables between subgroups suggests that these interventions should be tailored to the person’s demographic (age,sex and BMI) (III). Paper IV showed that even though the prevalence of obesity among Swedish women has increased substantially during these ten years, it has not kept pace with the increase in the US. One explanation for this may be that normal 30 year old Swedish women have more healthy behaviours than do US women. However, the insensitivity of the Swedish women to weight gain for healthy versus unhealthy alternatives may also be a factor. If the exact reason behind this phenomenon can be identified this may contribute to a deeper understanding of PWM both in Sweden and the US.