Diet, physical activity, and weight may affect prognosis among women who are diagnosed with breast or gynecologic cancer. Observational studies show associations between being overweight or obese and weight gain with several measures of reduced prognosis in women with breast cancer and some suggestion of poor prognosis in underweight women. Observational studies have shown an association between higher levels of physical activity and improved breast cancer–specific and all-cause mortality, although a dose-response relationship has not been established. One large randomized controlled trial reported increased disease-free survival after a mean of 5 years in patients with breast cancer randomly assigned to a low-fat diet versus control. However, another trial of similar size found no effect from a high vegetable/fruit, low-fat diet on breast cancer prognosis. The few reported studies suggest that obesity negatively affects endometrial cancer survival, while the limited data are mixed for associations of weight with ovarian cancer prognosis. Insufficient data exist for assessing associations of weight, physical activity, or diet with prognosis in other gynecologic cancers. Associations of particular micronutrient intake and alcohol use with prognosis are not defined for any of these cancers. The effects of dietary weight loss and increase in physical activity on survival or recurrence in breast and gynecologic cancers are not yet established, and randomized controlled trials are needed for definitive data.