OBJECTIVE: To identify the factors associated with weight gain after diagnosis of breast cancer in a heterogeneous population of women. DESIGN: Descriptive cross-sectional study. SUBJECTS: 1,116 patients who had been diagnosed with stage I, stage II, or stage IIIA primary, operable breast cancer within the previous 4 years. Patients were recruited during enrollment into a diet intervention trial to reduce risk for breast cancer recurrence. Analysis Demographic data, weight history, and physical activity information obtained by questionnaire and medical information obtained by chart review; dietary assessment based on four 24-hour dietary recalls collected by telephone. Associations between weight change after the diagnosis of breast cancer and prediction variables were examined using univariate and multiple linear regression analyses. RESULTS: Overall, 60% of the subjects reported weight gain, 26% reported weight loss, and 14% reported no change in weight after the diagnosis of breast cancer. The overall mean weight change was a gain of 2.7 kg (6 lb). Factors positively and independently associated with weight gain were time since diagnosis of breast cancer, adjuvant chemotherapy, African-American ethnicity, current energy intake, and postmenopausal status at time of study entry. Factors inversely and independently associated with weight gain were prediagnosis body mass index, age at diagnosis, education level, and exercise index score. APPLICATIONS: Higher energy intake and lower level of physical activity are independently associated with increased risk for weight gain after the diagnosis of breast cancer. Strategies to modify these behaviors are likely to influence the long-term pattern of weight change.