Etoposide, a semisynthetic derivative of podophyllotoxin, was evaluated concurrently in vitro against a human derived hormone-resistant cell line, PC-3, and in vivo in bidimensionally measurable hormone-resistant human prostatic cancer. In vitro, a dose-response relationship was observed, with 74% inhibition at 10 micrograms/ml (1 h incubation) and greater than 99% inhibition at 90 micrograms/ml, both in the range of clinically achievable concentrations. In vivo, 1 PR (5%, 95% confidence limits 0-12%) of 18+ months was observed in 20 adequately treated patients. The results confirm the limited role of etoposide in hormone-refractory disease and the need for new model systems for evaluation of potential chemotherapeutic compounds in this disease.