Asian men were genotyped for alleles of aldehyde dehydrogenase (ALDH2) and tested on two separate occasions following oral administration of placebo and 0.75 ml/kg alcohol. Sixty minutes after beverage ingestion, event-related potentials were elicited using an auditory oddball paradigm. Repeated measures ANOVA revealed that alcohol produced significant increases in P300 latency and significant decreases in P300 amplitude compared with placebo. Subjects with ALDH2*1/2*2 genotype (n = 14) demonstrated some significantly greater P300 effects after alcohol than subjects with ALDH2*1/2*1 genotype (n = 15), despite equivalent blood alcohol concentrations. These data suggest that neurocognitive functioning may be more impaired following alcohol in subjects with an ALDH2*2 allele. These findings further suggest that a genetically controlled factor (deficiency in ALDH enzyme activity) might contribute to a decreased likelihood of alcohol intake and protection from alcoholism, because of an enhanced sensitivity to alcohol.