PURPOSE: The purpose of this study was to determine the effects of eating breakfast on energy intake, physical activity, body weight, and body fat in women who were nonhabitual breakfast eaters over a one-month period. METHODS: We tested 49 premenopausal, nonhabitual breakfast-eating women to compare the effects of eating breakfast versus not eating breakfast. Each participant was randomized to one of two conditions: breakfast or no breakfast. Breakfast eaters were required to eat within an hour and a half of awakening and had to be finished eating their breakfast meal by 8:30 A.M. Non-breakfast eaters were defined as not consuming a snack or meal (with the exception of water) until after 11:30 A.M. Participants assigned to the breakfast condition consumed at least 15% of their daily energy requirement for breakfast. Weight and body fat were assessed at the baseline and after one month of intervention. Body fat was measured by dual-energy x-ray absorptiometry (DXA). Participants completed seven 24-hour recalls to assess dietary intake during the intervention. Physical activity was measured by accelerometry for 32 consecutive days. RESULTS: On average, the participants randomized to eat breakfast consumed 266 ± 496 (F = 12.81; P = 0.0043) more calories per day over the course of the study and weighed 0.6 ± 0.81 kg (F = 7.81; p = 0.0076) more at the end of the intervention. There was no observed caloric compensation at subsequent meals and no change in self-reported hunger or satiety. There was also no physical activity compensation with the addition of breakfast. CONCLUSION: The findings of our study showed that requiring non-breakfast eaters to eat breakfast resulted in higher caloric intake and weight gain. Future research should evaluate this relationship for a longer period of time to see if adding breakfast to the diet of women who generally do not eat breakfast results in adaptive behavior change over time.