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.