In 3,377 men and 3,900 women who participated in a community-based plasma cholesterol screening program, we found a significant cyclic time-trend in cholesterol levels, with maximum peak in January. The 95% confidence interval (CI) of the peak to trough distance was 5.8-13.8 mg/dL (0.15-0.36 mmol/L) in men, corresponding to 2.6%-6.3% of the average cholesterol level. Corresponding figures for women were 2.0-9.3 mg/dL (0.05-0.24 mmol/L) or 1.0%-4.6%. Applying the cutoff level for high cholesterol risk proposed by the National Cholesterol Education Program (≤ 240 mg/dL [6.21 mmol/L]) to sex-specific bimonthly distributions, we found a statistically significant variation in prevalence, attributable to seasonal trends, in men (P <.01), but not in women. In men, the age-adjusted prevalence in winter (25.4%) was double that in the summer (13.5%). Seasonal variation is an important determinant of the prevalence of hypercholesterolemia in men and should be considered in patient follow-up and screening.