To assess the efficacy of carboplatin in patients with hormone-refractory prostate cancer in terms of response rate and palliation, the Swiss Group for Clinical Cancer Research (SAKK) conducted this phase II clinical trial (SAKK 08/91). Carboplatin 400 mg/m2 was administered i.v. every 28 days to 27 patients. The prostate-specific antigen (PSA) level was monitored and compared with the clinical response. Tumour response was evaluated according to EORTC criteria. For patients with nonmeasurable disease, response was defined as the absence of progression in any tumour localization, with no increase in PSA and a decrease of at least 2 points in the WHO pain score. Selected aspects of quality of life (QL) and use of analgesics were assessed to describe patients' experience of toxicity and palliation. Only 1 patient with measurable and 2 patients with nonmeasurable disease achieved partial remission or a response according to our criteria. However, 13 of the 27 evaluable patients had some benefit from carboplatin therapy, as indicated by an improvement in performance status, reduction of pain, and stabilization of metastases. There was no clear-cut association between clinical response and PSA level. QL data suggested that carboplatin was relatively well tolerated and confirmed the clinically documented palliation. In particular, from baseline, for at least two consecutive cycles 7 patients reported either an improvement in pain by 1 point or more on a 4-point scale (> or = 33%) without an increase in analgesic intake or a decrease by 50% or more in analgesic intake without an increase in pain. With the dose and schedule used in this study, carboplatin had only limited objective activity in advanced prostate cancer, but induced palliation in about half the patients.