The interim results of the randomised, Phase III trial of Zoladex against castration in the management of patients with metastatic carcinoma of the prostate is discussed. Trials commenced in October 1984 and incorporated 359 patients when recruitment ceased in January 1986. The preliminary report concerns the first 240 patients who had a minimum of 3 months follow-up. Entry criteria included patients who had no previous treatment with the exception of first-line localised radiotherapy and those who had distant bone or soft tissue metastases. Fourteen patients were excluded on the basis of protocol violations. The objective assessment of response was based on the British Prostate Group Criteria and was performed monthly for the first 3 months and 3-monthly thereafter. Pre-treatment disease characteristics of patients in both groups were similar at entry and there were no significant differences in the subjective response data of patients between the orchidectomy (n = 106) and the Zoladex group (n = 120). Objective response rates at 12 and 24 weeks of treatment were also identical for both treatment groups. Serum testosterone concentrations were below the 'castrate' level (less than 2 nmol/L) for Zoladex group as well as the orchidectomy group up to 48 weeks. The drug was well-tolerated with minimal side effects, those resulting from testosterone withdrawal were similar in both groups. The report therefore indicates clearly that this partical formulation of LH-RH analogue provides a valuable alternative to the surgical procedure in the treatment of carcinoma of the prostate.