Background: In prostate cancer (PCa) treated with androgen deprivation therapy (ADT) and radiotherapy (RT), limited prospective data exist regarding weight variation during ADT. Using our prospective data from two phase III trials we compare the magnitude of weight variation during different duration of ADT. Methods: 400 patients (pts) with intermediate risk PCa received 6 months of ADT (Bicalutamide 1 month (m) and Goserelin 10,8 mg x 2), and 630 pts with high-risk PCa were treated with 18 or 36m of ADT (Bicalutamide 1m and Goserelin 10,8 mg x 6 or 12). Pts’ weights were measured at baseline then at each Goserelin dose. 463/1030 pts were excluded from the analysis: 90 no or incomplete ADT, 127 no initial weight and 246 no weight at the end of ADT. Regular patient weighing was available in 567/1030 pts (55%) from whom 250, 191 and 124 received, respectively, 6, 18 or 36m of ADT. Pt’s characteristics and weights were compared with Kruskal-Wallis and Chi squared test between groups of ADT. Multivariable logistic regression was performed to assess predictors of weight gain over 2 kg. Variables included were duration of ADT, age ≥65 vs. < 65 and baseline comorbidities (cardiovascular disease, diabetes, chronic obstructive pulmonary disease, hypertension). Results: Comparing weights between the beginning and the end of ADT, 375/567 (66.1%) pts gained and 192/567 (33.9%) lost weight. The mean ± SD weight gain at the end of ADT was significantly higher in pts receiving 18m of ADT (1.98 ± 4.05) or 36m (2.38 ± 5.15) compared to pts receiving only 6m (0.98 ± 3.10, p=0.003). As ADT duration increased (6, 18 or 36m), weight gain over 2 kg (34.1% vs. 48.2% vs. 53.2%, p<0.001) and over 4 kg (12.7% vs. 26.2% vs. 36.3%, p<0.001) significantly increased. In multivariable logistic regression, predictors of weight gain over 2 kg were 18m ADT [OR=1.85 (1.25-2.74), p=0.002] and 36m ADT [OR=2.23 (1.43-3.48), p<0.001] compared to 6m ADT and non-diabetic pts (OR=2.13 (1.28-3.54), p=0.003). Age was not a factor. In the 375 pts who gained weight, the rate of patients that gained 0.1 to 2 kg, 2.1 to 4 kg and more than 4 kg were respectively (47.2%, 33.1% and 19.6%) in the 163 receiving 6m ADT, (30.3%, 32.6% and 37.1%) in 132 pts receiving 18m ADT, and (17.6%, 26.3% and 56.4%) in the 80 pts receiving 36m ADT. In the 192 pts who lost weight, the rate of patients that lost 0.1 to 2 kg, 2.1 to 4 kg and more than 4 kg were respectively (62.9%, 21.3% and 15.6%) in 89 receiving 6m ADT, (50.8%, 27.1% and 22.1%) in 59 pts receiving 18m ADT and (59.1%, 15.9% and 25%) in 44 pts receiving 36m ADT. Conclusions: ADT is not associated with weight gain in all pts. Based on our prospective data, one third of pts lose weight at the end of ADT. However, weight gain above 2 kg or 4 kg increases with ADT duration and non-diabetics pts, regardless of age. Weight monitoring in pts receiving ADT should be done regularly and pts counseled accordingly.