Abstract The goals in diabetes care are good metabolic control, minimisation of complications due to diabetes and a good quality of life. In order to achieve these goals, it is important that persons with diabetes adhere to self-care. Internationally, there are numerous studies available about adherence to self-care among adults with diabetes, but in Finland there are hardly any, especially in nursing science. The purpose of this study was to examine adherence to self-care and also how metabolic control, social support and health locus of control beliefs are related to patient adherence. The data were gathered by questionnaires from 213 working-aged adults with insulin-treated diabetes from the Oulu Health Center or the Central Hospital of Lapland. The response rate was 76%. In order to verify the reliability and the validity of the instruments, we used correlation coefficients, factor analysis and item-total analysis. Internal consistency was checked by Cronbach's alfa. Cross-tabulations with the chi-square test of independence, correlations, One- and Two-Way ANOVA and covariate analysis were used for data analysis. Our multivariate statistical methods consisted of logistic and multiple linear regression analysis and cluster analysis. The subjects who were adherent to self-care had better metabolic control than those who neglected self-care. A fifth of the respondents were neglecting their self-care. The others undertook flexible, regimen-adherent or self-planned self-care. Poor metabolic control, smoking and living alone explained neglect of self-care. If the diabetic got support from her/his family and friends, living alone was not a predictor of neglect of self-care. Those who were adherent to self-care perceived themselves as getting more support from their family and friends than the group who neglected self-care. Those who had poor metabolic control perceived themselves as getting peer support from other persons with diabetes. Those who were adherent to self-care were responsible internals who believed both in their own action and in health care team action. Those who mainly believed in health care team action (powerful other externals) were more adherent to self-care than those with an internal or chance external health locus of control. We got evidence about the importance of adherence to self-care, and its relationship with metabolic control, social support anf HLOC beliefs. The results can be used when developing the individual patient education of persons with diabetes.