BACKGROUNDInitial treatment with a gonadotropin‐releasing hormone (GnRH) agonist increases fat mass, decreases lean body mass, and decreases bone mineral density (BMD) in men with prostate carcinoma. To the authors' knowledge, little is known regarding either the long‐term effects of treatment or predictors of treatment‐related changes in BMD and body composition.METHODSThe authors analyzed prospective 12‐month data from 65 men during initial and long‐term GnRH agonist treatment for prostate carcinoma. Relationships between baseline characteristics (age, treatment duration, body mass index, and baseline values for outcome of interest) and changes in lean mass, fat mass, and BMD were assessed using univariate and multivariate regression models.RESULTSMean BMD of total hip decreased by 1.9 ± 2.7%, lean body mass decreased by 2.0 ± 3.3%, and fat mass increased by 6.6 ± 9.4% at 12 months (P < 0.001 for each comparison). Twenty‐three men (35%) had received treatment with a GnRH agonist before study entry, and the mean (± standard deviation) duration of previous treatment was 35 ± 41 months. Longer duration of previous GnRH agonist treatment was found to independently predict less fat accumulation and less loss of lean body mass in multivariate models. In contrast, the duration of GnRH agonist treatment did not significantly predict changes in BMD. Other covariates did not appear to predict changes in body composition or BMD consistently.CONCLUSIONSThe results of the current study showed that fat mass increased and lean body mass decreased mostly during initial GnRH agonist therapy whereas BMD decreased steadily during initial and long‐term treatment. Cancer 2005. © 2005 American Cancer Society.