Eighty-five patients treated by radical prostatectomy for clinically localised prostatic cancer were followed up for 1 to 4 years with measurement of prostate specific antigen (PSA). Six patients with recurrences had elevated levels (cut-off level was 1.0 ng/ml). PSA is therefore considered an excellent tool for monitoring treatment failures. Levels exceeding 1.0 ng/ml preceded evidence of tumour recurrence by a mean interval of 11 months. PSA offers the possibility of detecting residual prostatic cancer after surgery. It is not known, however, whether these patients would have benefited from adjuvant endocrine or early radiotherapy.