Serum samples from 954 Irish adults (604 males, 350 females) aged 18 to 65 years were analyzed within 24 hours of receipt for non-fasting total serum cholesterol levels. The subjects were volunteer blood donors (Blood Transfusion Service Board, Pelican House, Dublin) presenting from June 1990 to February 1991. Serum cholesterol increased significantly with age in both males and females. Mean serum cholesterol in males increased from 4.5 (+/- 0.9) mmol/l in those < 25 years, to 5.7 (+/- 1.0) mmol/l at age 55-64 years. In females, mean serum cholesterol increased from 4.3 (+/- 0.9) mmol/l less than 25 years to 6.2 (+/- 0.8) mmol/l at age 55-64 years. Options for reporting cholesterol ranges are discussed. The overall mean cholesterol concentration of 5.1 mmol/l seen in this study is lower than that reported for comparable studies of U.K. and American subjects, but higher than observed in Japanese individuals. Almost 48% of the studied population had a serum cholesterol value less than 5 mmol/l and over 56% had levels less than 5.2 mmol/l.