Introduction The use of androgen deprivation therapy (ADT) in the treatment of prostate cancer is associated with changes in body composition including increased fat and decreased lean mass. Limited information exists regarding the rate and extent of these changes. This systematic review was conducted to determine the effects of ADT on body composition in prostate cancer patients. Methods Literature searches were conducted on MEDLINE, EMBASE and Web of Science for studies until January 2009. Only longitudinal studies that examined ADT and body composition in prostate cancer patients were included. Data were extracted on body weight, BMI, percentage of fat mass and lean body mass. Results Sixteen studies (14 cohorts and 2 RCTs) met the inclusion criteria. Pooled data, calculated according to a random effects model, showed that ADT increased % body fat by on average 7.7% (95% CI 4.3, 11.2, from seven studies, P < 0.0001) and decreased % lean body mass by on average −2.8% (95% CI −3.6, −2.0, from six studies, P < 0.0001) but for both there was marked heterogeneity between studies (I2 = 99% I2 = 73%, respectively). Similarly, body weight (2.1%, P < 0.0001 from nine studies) and BMI (2.2%, P < 0.0001, from eight studies) increased significantly. More extensive changes were seen with longer duration of treatment. Conclusions Substantial increases in fat and declines in lean mass were observed in prostate cancer patients treated with ADT. Lifestyle changes or suitable interventions to minimize the effect of ADT on body composition need to be investigated. Implications for cancer survivors Prostate cancer survivors should be made aware of the side effect of treatment on body composition and further work is required to determine what interventions can minimize the impact of ADT on body composition and therefore what evidence based advice they should be provided with. In general, though recommendation of a healthy diet and moderate exercise is reasonable.