The Prostate Cancer Prevention Trial is an intergroup effort in the USA managed by the Southwest Oncology Group (SWOG) in collaboration with the Eastern Cooperative Oncology Group (ECOG) and the Cancer and Leukemia Group B (CALGB). This 10-year study began approximately 5 years ago and will achieve its primary endpoint in October 2004. At the start of the study, 18,882 men, aged over 55 years, and with normal digital rectal examination (DRE) and serum prostate-specific antigen (PSA) levels of 4.0 ng/ml, a biopsy is recommended. Because of the effect finasteride has on PSA, the PSA value has been indexed to equalize the number of biopsies in both arms. At 7 years all survivors will undergo a sextant biopsy to determine the period prevalence of prostate cancer. The critical assumptions are: (1) finasteride-induced PSA changes result in a simple downward shift; (2) the assessment of adherence is sensitive enough to detect nonadherence affecting PSA level interpretation: (3) factors affecting biopsy loss will be equal in both arms; (4) finasteride does not affect the sensitivity or specificity of DRE on transrectal ultrasound nor the sensitivity of biopsy; (5) bias resulting from transurethral resection of the prostate in benign prostate hyperplasia cases will be negligible.