Although commonly thought to be an effective method of screening for prostatic cancer, digital rectal examinations have yet to be shown by controlled study to help prevent advanced forms of this disease. 139 men with metastatic (stage D) prostatic cancer (cases) were compared with an equal number of matched men free of this condition (controls), with respect to rectal examinations recorded in the medical records up to, on average, 23 years before the cases' initial diagnosis of prostatic cancer. Cases and controls were members of a large health maintenance organisation in northern California. In the 10 years before initial diagnosis (excluding the last three months) the average number of examinations for routine screening (2.45 vs 2.52) or to evaluate intestinal or rectal symptoms (0.44 in both) were similar in cases and controls, respectively. After adjustment for racial differences, the relative risk of metastatic prostatic cancer for men with one or more screening rectal examinations compared with men with none was 0.9 with a 95% confidence interval of 0.5-1.7. Screening by routine digital rectal examination appears to have little if any effect in preventing metastatic prostatic cancer. If there is a small benefit, it will be difficult to demonstrate by conventional epidemiological study.