There is a pressing need for new agents for the treatment of hormone-resistant prostate cancer (HRPC). Pyrazoloacridine (PZA) has antitumor activity in several in vitro and in vivo tumor systems, and was selected for testing in clinical trials by the National Cancer Institute (NCI). We conducted a phase II trial of PZA for the treatment of HRPC. Seventeen male patients with HRPC were treated with PZA at 750 mg/m2 i.v. given over a period of 3 hr every 3 weeks. Response to therapy was assessed with serial measurements of serum prostate-specific antigen (PSA) and sequential imaging studies. The 17 patients were treated and fully evaluable. One patient experienced a significant decrease in PSA, from over 10,000 ng/ml to 423 ng/ml, along with an improvement in bone scan findings. However, no other patient obtained an objective or PSA response (overall PSA response rate = 5.9%). Median survival duration was 15.3 months. Toxicity was moderate. If PSA is used as a marker of response, single-agent PZA appears to lack efficacy in the treatment of HRPC. However, the one unambiguous response, and the favorable toxicity profile observed, may warrant further evaluation of this agent.