Aim: Aldehyde dehydrogenase 1 (ALDH1) has been advocated as a marker of alcohol intake. The absence or low levels of ALDH1 may be associated with alcohol-induced flushing or other reactions to alcohol in Europeans and therefore, with reduced alcohol use. This study tested whether variation in erythrocyte ALDH1 activity was associated with alcohol use, alcohol dependence or reactions to alcohol in unselected subjects of European descent, and whether variation in ALDH1 activity was subject to genetic influences. Methods: ALDH activity was measured in erythrocytes from 677 men and women who had participated in a twin study of alcohol use and dependence. Results: There were no significant effects of sex, alcohol consumption or alcohol dependence on ALDH activity. Subjects who reported reactions to alcohol did not have low activity. Women aged below 45 years had lower ALDH activity than men or older women. The heritability of ALDH activity was 56% (95% confidence interval = 42-67%). Conclusions: Previous reports that erythrocyte ALDH activity is low in alcoholics were not substantiated in this community-based sample. Associations with alcohol reactions were not found. ALDH activity varies widely between subjects, largely because of genetic factors.