Acute lung function response to cotton dust exposure was studied in atopic and non atopic humans. A total of 226 healthy, non asthmatic adults, with an average age of 29 from a South Carolina community, was exposed to two separate 6 hour exposures of cotton dust at concentrations of 1.02 milligrams per cubic meter. None of the subjects were currently employed in the cotton textile industry. The subjects were administered spirometric tests immediately before and after each dust exposure. The subjects were administered prick skin tests consisting of pollens from grasses, weeds, and trees, Alternaria-tenuis, house dust, and mite. Atopy, defined as positive prick test response to at least two allergens, was observed in 26 percent of the subjects. Eight percent of the subjects had a positive response to two allergens and 18 percent to three or more allergens. Mite, ragweed, and Kentucky blue grass caused the highest rates of reactivity. Significant exposure related decreases occurred in all spirometric tests independent of atopic status. Cotton induced decreases in lung function were generally greater in atopic individuals; however, only the 1 second forced expiratory volume (FEV1) was significantly lower. Degree of atopy, as measured by the number of positive skin test responses, was significantly correlated with cotton induced decreases in FEV1 values. The authors conclude that atopy may be an important factor in determining the magnitude of the acute pulmonary response to cotton dust.