Screening for prostate cancer at low prostate-specific antigen (PSA) levels (<or=4.0 ng/mL) risks detecting clinically insignificant cancers, which are of no threat to the man. In this review we evaluate the prevalence and tumour characteristics of prostate cancer detected at low PSA levels, comparing screening studies, cystoprostatectomy series and autopsy data. The favourable characteristics of tumours detectable at very low PSA levels seem to justify the conclusion that an unknown but sizeable proportion of the cancers found at biopsy are clinically insignificant.