PURPOSE: Changes in cancer therapy, in addition to changes in obesity prevalence, suggest the need for a current assessment of weight gain patterns following breast cancer diagnosis. The aim of this study was to evaluate factors associated with weight gain among breast cancer survivors prior to enrolling into a behavioral weight loss intervention. METHODS: Anthropometric measures and data on weight-related factors were collected at baseline on 665 breast cancer survivors. Postdiagnosis weight gain was determined between entry into the trial and previous diagnosis up to 5 years. Multivariate logistic regression analyses were used to evaluate the association between weight gain and influencing factors. RESULTS: The mean weight gain was 4.5 % body weight (standard deviation = 10.6); 44 % of women experienced ≥5 % body weight gain. The risk of weight gain was inversely associated with age (adjusted odds ratio (ORadj) = 0.97, 95 % confidence interval (95 % CI) 0.95-0.99), Hispanic ethnicity (ORadj = 0.30, 95 % CI 0.13-0.68), and overweight (ORadj = 0.11, 95 % CI 0.05-0.23) or obese (ORadj = 0.03, 95 % CI 0.02-0.07) status at diagnosis and positively associated with time elapsed since diagnosis (ORadj = 1.19/year, 95 % CI 1.04-1.36). Women prescribed aromatase inhibitors were 46 % less likely to gain weight compared to women prescribed selective estrogen-receptor modulators (ORadj = 0.54, 95 % CI 0.31-0.93). The risk of weight gain was positively associated with smoking at diagnosis (ORadj = 2.69, 95 % CI 1.12-6.49) although this was attributable to women who subsequently quit smoking. CONCLUSIONS: Postdiagnosis weight gain is common and complex and influenced by age, ethnicity, weight, smoking status, time elapsed since diagnosis, and endocrine-modulating therapy. IMPLICATIONS FOR CANCER SURVIVORS: Weight gain continues to be a concern following a diagnosis of breast cancer. Factors influencing this weight gain include age, ethnicity, weight, smoking status, time elapsed since diagnosis, and endocrine-modulating therapy. Effective weight management strategies are needed for this population of women.