PURPOSE: Hot flashes (HF) affect a large proportion of breast cancer (BC) survivors and can negatively affect their quality of life. Treatments other than estrogen replacement to alleviate HF are needed. Body weight is related to hot flashes, but little is known about the effect of weight change on HF. PATIENTS AND METHODS: We used data from 3,088 women previously treated for early-stage BC who were enrolled onto the Women’s Healthy Eating and Living study to examine the association between weight change after a breast cancer diagnosis and the odds of reporting HF. RESULTS: Overall, 36.1% of participants reported moderate to severe HF at study entry. At 2 years postdiagnosis, 69.2% of women remained within 10%, 4.8% lost at least 10%, and 26.0% gained at least 10% of their prediagnosis weight. Those who gained at least 10% of their prediagnosis weight had a greater risk of reporting HF than women who remained weight stable in that same period (odds ratio [OR], 1.33; 95% CI, 1.11 to 1.60; P = .003). Weight loss of at least 10% of prediagnosis weight was associated with a nonsignificant reduced risk (OR, 0.72; 95% CI, 0.47 to 1.08; P = .118) of reporting HF. However, the trend of weight change (weight loss and weight gain) on HF was significant both when examined categorically (P = .03) and continuously (P < .001). CONCLUSION: Prevention of weight gain after a BC diagnosis-a modifiable behavior-may offer a viable intervention for relief of HF. Effects of intentional weight loss in BC survivors requires further study.