The independent prognostic factors affecting survival were assessed in 240 men undergoing treatment for metastatic prostate cancer as part of a randomized clinical trial comparing the gonadotropin releasing hormone analogue Zoladex (goserelin acetate implant) with castration. In a multivariate analysis, the most highly significant predictors were the presence or absence of bone pain, serum testosterone levels, serum alkaline phosphatase levels, and performance status. Patients with all four factors favorable for survival had a 2-year survival rate of 84% as compared with only 8% for patients with none of the four factors favorable for survival. No other factors were significant. A separate analysis of serum testosterone levels revealed that the higher the pretreatment serum testosterone level, the greater the survival rate. Compared with patients with serum testosterone levels less than 6.9 nmol/L, significant differences in survival were observed for patients with serum testosterone levels of 10.4 to 13.9,13.9 to 17.3, and over 17.3 nmol/L. These results have important implications for the design and analysis of future clinical trials of hormone therapy and for counseling patients regarding the short-term prognosis of their disease.