New reliable methods for the determination of acetaldehyde in human blood, either from separated plasma or from acid-precipitated whole blood, demonstrate that the blood of healthy Caucasians contains at most only extremely small amounts of acetaldehyde (less than 1 microM) after moderate alcohol intoxication. On the other hand, among about 50% of the Japanese population ethanol ingestion results in elevated blood acetaldehyde levels (10-50 microM) with consequent unpleasant cardiovascular responses such as facial flushing and tachycardia, apparently because of a lack of one of the acetaldehyde-oxidizing aldehyde dehydrogenase isozymes. Elevated acetaldehyde levels may eventually occur also among intoxicated Caucasian alcoholics, primarily as a consequence of abuse-induced loss of hepatic aldehyde dehydrogenase activity, but accentuated by an accelerated ethanol oxidation rate. The elevation is probably reversible, since no acetaldehyde is seen in alcoholics after abstinence and hospital treatment. There is thus little evidence that an elevation of acetaldehyde could serve as a marker for predisposition for alcoholism.