The serum marker prostate-specific antigen (PSA) has been shown to be proportional to the volume of prostate cancer (Yang polyclonal assay). Therefore, changes in PSA with time in untreated patients should reflect tumor growth rate and the shape of the growth curve. Forty-three patients with untreated prostate cancer had serial PSA measurements over an average time span of 30 months. The increase of PSA was exponential (log-linear) throughout the measured interval. Doubling times were faster in patients with higher stages and grades. Twenty of 28 cancers thought to be clinically organ confined doubled at rates exceeding 4 years. Tumor doubling times were overestimated in patients with large volume benign prostatic hyperplasia since hyperplasia increases serum PSA, albeit 12 times less than cancer. We conclude that prostate cancer has a constant (log-linear) growth rate which is very slow. This slow doubling time has substantial consequences for therapeutic decisions and for screening programs.