This study measures the amount of body sway or static ataxia in 34 drinking but nonalcoholic men 21 to 25 years of age who have an alcoholic first-degree relative (the family-history-positive, or FHP, group). Results are compared with 34 control subjects matched pairwise on demographic characteristics and drinking histories, but who have no known alcoholic close relatives (the family-history-negative, or FHN, group). Each man was tested on three occasions where he drank either placebo, or 0.75 mL/kg or 1.1 mL/kg of ethanol; the subjects were repeatedly tested during the subsequent four hours. At the baseline of each of the three test sessions, the level of body sway for the two family-history groups was virtually identical. However, following the 0.75-mL/kg dose, the increase in body sway was significantly less for the FHP than for FHN group, with similar but less dramatic group differences noted following the ingestion of 1.1 mL/kg of ethanol. These results are consistent with the significantly less intense subjective feelings of intoxication after drinking for the FHP men, and also parallel findings of less intense ethanol-related changes in biologic and cognitive test scores. A decreased intensity of reaction to ethanol should be explored further as a possible genetic trait marker of a predisposition toward alcoholism.