Phosphatidylethanol (PEth) is formed only in the presence of ethanol, via the action of phospholipase D. We studied PEth in blood as a possible marker of alcohol abuse in 15 male alcoholics admitted for detoxification. Blood was drawn on the first day after admission and up to 28 days thereafter. PEth in whole blood was 13.2 +/- 2.2 mumol liter-1 (mean +/- SE) at first sampling and remained detectable up to 14 days after admission. Blood ethanol was 0 on the morning after admission. The time courses of PEth disappearance varied among individuals. No PEth could be found in blood of control persons who had abstained from ethanol for 4 days. Levels of PEth and carbohydrate-deficient transferrin or gamma-glutamyltranspeptidase did not correlate. Its high specificity and prolonged detectability suggest PEth in blood as a marker of recent alcohol abuse.