Hot flashes in breast cancer survivors (BCS) receiving adjuvant aromatase inhibitor (AI) therapy are common, but risk factors for these symptoms are ill-defined. This study tested if body size is associated with hot flashes in BCS on AI therapy. A cross-sectional study of postmenopausal BCS receiving adjuvant AI therapy was performed. The primary outcome was occurrence of patient-reported hot flashes. The primary exposures of interest were current body size and weight change since breast cancer diagnosis. Three hundred participants were enrolled at a mean age of 61 years (range 33-86) after an average AI exposure of 23 months (range 1 month-9 years). Fifty-nine percent reported hot flashes, 32% reported moderate to severe hot flashes, and 25% reported significant worsening of hot flashes since starting AI therapy. Sixty-one percent experienced weight maintenance (±10 lb), while 27% had weight gain (gained 10 lb or more), and 11% had weight loss (lost 10 lb or more). In multivariable analysis, weight gain was independently associated with hot flash occurrence (OR 2.1, 95% CI 1.1-4.4) and hot flash severity (OR 2.6, 95% CI 1.3-5.0) after adjusting for confounding. Current body size was not associated with hot flash occurrence, severity or change with AI therapy. In an outpatient BCS population on AI therapy, weight gain is a risk factor for hot flash occurrence. Women who gained at least 10 lb since breast cancer diagnosis were two times more likely to have hot flashes than women who maintained or lost weight. These results support the thermoregulatory model of hot flashes and argue against a protective effect of body fat in this population.