The intra- and interindividual variability of the oral glucose tolerance test (OGTT) and other risk factors was investigated in 237 subjects, aged 64-87, examined annually in the period 1971-1975. Coefficients of intraindividual variation (CVa) were calculated from individual regressions on time. The lowest CVa was found for the summary index including fasting glucose (area under the curve, AUC): 10.0 +/- 4.9%. For fasting and 30, 60, and 120 min glucose the values ranged from 12 to 18%. The CVa's were not associated with age, gender, drug use, and disease prevalence, and may also be applied to other populations. The reliability coefficient depended on the prevalence of diabetes in the population and was higher than observed in younger populations. The highest reliability coefficient was observed for AUC: 0.81. For the combined information of OGTT, reflected by AUC or by classification according to WHO criteria, the variability was comparable to that of other cardiovascular risk factors such as serum total cholesterol.