In recent years, a number of investigators have studied the relationship between IGF-I and risk of developing cancer, diabetes or cardiovascular disease. Upper tertile, quartile, and quintile IGF-Is were associated with higher risk of developing cancer, and lowest quartile with cardiac disease and diabetes. As part of a study to correlate serum IGF-Is and growth hormone dynamics in aging, we measured fasting serum IGF-I at baseline and two weeks later in a group of 84 normal volunteers between the ages of 50 and 90 years. Although the correlation between the two IGF-Is was high (r=0.922; p<0.0001) there were substantial differences between the two IGF-I values ranging from -36.25 to +38.24% between individual IGF-I values at the two blood draws and a significant difference between the mean IGF-Is at visits I and 2 (mean 120.28+/-53.5 vs. 114.95+/-50.03; p=0.03). When considered in quartiles, IGF-I changed from one quartile to another in 34/84 (40.5%) of the volunteers. When the group was divided in halves, tertiles,quartiles, or quintiles there was an increasing number of subjects who changed from one subdivision to another as the number of gradations increased. These results suggest that the predictive outcomes of earlier studies that used single IGF-I samples for analysis of risk ratios according to tertiles, quartiles, or quintiles could have been different if a second IGF-I was used to establish the risk ratio. The results also suggest that variability in IGF-I should be taken into account when designing such studies.