In preventing non-insulin-dependent diabetes mellitus (NIDDM) and its complications, screening high-risk individuals complements public health measures. Our screening instrument for patients of general practitioners was a questionnaire for self-determined high-risk groups plus a laboratory measurement of a random venous plasma glucose level. Collaborating practitioners evaluated 100 consecutive outpatients aged 40 years or older. The questionnaire identified patients with two or more diabetic symptoms or with two or more risk factors, and they were recommended to have their blood tested. For those with a random plasma glucose greater than 5.5 mmol/L, oral glucose tolerance tests (OGTTs) were advised. Of 50,859 subjects completing the study, there were 1,013 cases (2.0%) of new diabetes, 1,704 cases (3.4%) of impaired glucose tolerance (IGT), and 5,508 cases (10.8%) of previously diagnosed diabetes. Symptoms alone were a relatively poor discriminant. Almost all newly identified NIDDM and IGT patients had two or more risk factors for NIDDM. The risk ratios for abnormal glucose tolerance were as follows: high blood pressure, 2.4; overweight, 2.0; and positive family history, 1.7. Selection of cutoff points higher than 5.5 mmol/L would have substantially reduced the rate of newly discovered NIDDM and IGT. Screening for NIDDM and IGT in general practice is feasible and can be achieved with little disruption of office procedures. In preventive programs of this nature, the low screening threshold of 5.5 mmol/L for random venous plasma glucose maximizes the case-finding rate.