OBJECTIVE--To assess the reliability of the SF 36 health survey questionnaire in two patient populations. DESIGN--Postal questionnaire followed up, if necessary, by two reminders at two week intervals. Retest questionnaires were administered postally at two weeks in the first study and at one week in the second study. SETTING--Outpatient clinics and four training general practices in Grampian region in the north east of Scotland (study 1); a gastroenterology outpatient clinic in Aberdeen Royal Hospitals Trust (study 2). PATIENTS--1787 patients presenting with one of four conditions: low back pain, menorrhagia, suspected peptic ulcer, and varicose veins and identified between March and June 1991 (study 1) and 573 patients attending a gastroenterology clinic in April 1993. MAIN MEASURES--Assessment of internal consistency reliability with Cronbach's alpha coefficient and of test-retest reliability with the Pearson correlation coefficient and confidence interval analysis. RESULTS--In study 1, 1317 of 1746 (75.4%) correctly identified patients entered the study and in study 2, 549 of 573 (95.8%). Both methods of assessing reliability produced similar results for most of the SF 36 scales. The most conservative estimates of reliability gave 95% confidence intervals for an individual patient's score difference ranging from -19 to 19 for the scales measuring physical functioning and general health perceptions, to -65.7 to 65.7 for the scale measuring role limitations attributable to emotional problems. In a controlled clinical trial with sample sizes of 65 patients in each group, statistically significant differences of 20 points can be detected on all eight SF 36 scales. CONCLUSIONS--All eight scales of the SF 36 questionnaire show high reliability when used to monitor health in groups of patients, and at least four scales possess adequate reliability for use in managing individual patients. Further studies are required to test the feasibility of implementing the SF 36 and other outcome measures in routine clinical practice within the health service.