We studied a patient with alcoholic acidosis and an increased osmolal gap. Ethyl alcohol and other compounds that are known to increase serum osmolality in alcoholics were not detected. However, the levels of glycerol, acetone, and the acetone metabolites acetol and 1,2-propanediol were increased in the serum of this patient. On admission and 3 and 7 hours after admission, the combined serum osmolality of glycerol, acetone, acetol, and 1,2-propanediol accounted for 48%, 92%, and 62% of the increase in the osmolal gap above the highest normal level of 10 mOsm/kg H2O. The disappearance of the osmolal gap correlated with the correction of the acidosis and the concomitant reduction in serum glycerol and acetone levels. Elevations of endogenous glycerol, acetone, and acetone metabolite levels should now be added as causes for an increased osmolal gap in the alcoholic patient. Ingestion of toxic alcohols can no longer be assumed to be the only cause for an increased osmolal gap in alcoholic patients.