• Chemical determinations of serum cholesterol and phospholipid values were made in 1,200 specimens of blood obtained from healthy males and females between the ages of 2 and 77 years. The sample represented a particular geographic group with certain sociological characteristics. In 38 men of the 28 to 32 year age group, the average serum cholesterol value was 243 mg. per 100 cc.; the corresponding figure for 50 women was 200. A similar difference was found in the serum phospholipid values, and both differences were statistically significant. The differences were reversed in the older age groups; in 45 men of the 53 to 57 year age group, the average serum cholesterol value was 240 mg. per 100 cc., while in 28 women the corresponding figure was 286. Both the cholesterol and the phospholipid values for the males remained constant through age 19, increased from age 20 through 33, and then remained constant to age 60; for females they remained practically constant through age 32, then increased sharply through age 58. The reversal of the difference between sexes suggests an explanation of the known preponderance of males among patients under 50 suffering from disease of the coronary arteries.