Available information suggests that individuals with breast cancer gain weight during adjuvant treatment and that this weight gain may be associated with poor prognosis. Exploration of the factors which affect weight gain may aid in developing weight control interventions for these patients. To determine the factors which are associated with weight gain, 32 women undergoing adjuvant chemotherapy were followed over 2 years from the beginning of adjuvant treatment. Measures of psychologic functioning and self-reports of exercise levels and eating were assessed every 2 months during the course of treatment. Sixty-nine percent of the women gained weight over treatment, resulting in a significant weight gain for the group as a whole. Weight gain was correlated positively with several psychologic measures but not with assessed biologic measures. A multiple-regression equation using psychologic/behavioural measures of emotional discharge, logical analysis, affective regulation, interpersonal sensitivity, average number of symptoms, and obsessive compulsiveness accounted for 58% of the variance in overall weight gain. At 2 years of follow-up, 27 women had gained weight for an average of 6.03 kg. The coping style of logical analysis emerged as a significant predictor of disease recurrence, accounting for 28% of the variance in weight gain at 2 years. The results are discussed in terms of identification of women likely to gain weight during adjuvant treatment, directions for future research, and development of interventions to control weight gain.