A group consisting of 153 elderly patients with arteriosclerotic vascular diseases was investigated for C3 phenotypes by means of high-voltage starch gel electrophoresis, and the C3 type distribution was compared to the distribution in a group consisting of 758 young Danish persons involved in paternity cases. A preponderance of C31 genes and a deficiency of C32 genes were found among the old arteriosclerotic patients. The difference between the two samples is statistically significant. The probability of the difference being due to chance is less than 1 in 1,000.