OBJECTIVES: To establish age- and sex-specific reference limits for the erythrocyte sedimentation rate (ESR) in asymptomatic Norwegian adults. DESIGN: Single ESR recordings were obtained by the classical or a modified Westergren method from 2145 men and 1765 women (93% being blood donors) with age range 20-90 years, and analysed statistically. RESULTS: There was a significant positive association between ESR level and age, consistent with a parabolic pattern in men but a linear one in women. The mean values for men were about 3 mm h-1 at 20 years, 6 mm h-1 at 55 years, and 10 mm h-1 at 90 years, and 6, 9, and 11 mm h-1 respectively for women. These averages (predicted by regression lines) were significantly higher in women up to the age of 75 years, after which the estimated sex-specific 95% confidence limits for mean values were found to overlap. CONCLUSIONS: The upper reference levels expected to be exceeded only by chance in 5% of single individual recordings at the ages of 20, 55 or 90 years, respectively, were estimated to be 12, 14 and 19 mm h-1 for men, and 18, 21 and 23 mm h-1 for women. Higher values should be controlled and, if confirmed, lead to a clinical check-up. However, about 76% of our overall material had ESR values lower than 9 mm h-1. Knowledge of each person's baseline ESR value might increase the disease-predictive ability of the test. If several measurements over years reveal a steeper rise with age than depicted in our population-based curves, it should be taken seriously, even when each reading is below the population-based reference limits.