Endogenous (fasting) breath acetaldehyde levels were assessed as a discriminator between alcoholic and non-alcoholic subjects. The influence of smoking as a potential confounding variable on breath acetaldehyde was determined. Individuals with a history of smoking and/or drinking had higher endogenous breath acetaldehyde levels as compared to controls. Mean acetaldehyde levels revealed an additive (noninteractive) pattern associated with combined abuse of these substances. The similarity in acetaldehyde levels between alcoholic non-smokers and non-alcoholic smokers limits the usefulness of endogenous breath acetaldehyde for the purpose of early diagnosis of alcoholism. Results support hypotheses that elevations in acetaldehyde could be a common factor in disease associated with alcohol and cigarette abuse.