Recent studies have shown enhanced responsiveness to ozone in obese mice. Adiposity has not been examined as a possible modulator of ozone response in humans. We therefore examined the relationship between body mass index and the acute spirometric response to ozone (O(3)) exposure among 197 nonasthmatic young adults (aged 18-35 yr) studied in our human exposure facility from 1992 to 1998. Each subject had been exposed to 0.42 ppm O(3) for 1.5 h with intermittent exercise designed to produce a minute ventilation of 20 L/min/m(2) body surface area (BSA). Spirometry (pulmonary function) was measured pre- and immediately postexposure to determine acute ozone-induced changes. The decrement in forced expiratory volume in 1s (Delta FEV1) as percent of baseline was significantly correlated with BMI, r = -0.16, p = .03, with a slightly stronger correlation in women (n = 75), r = -0.22, p = .05, and no significant correlation in men. BMI had a greater range in women than in men in our study. In women greater ozone-induced decrements were seen in overweight (BMI > 25 kg/m(2)) than in normal weight (BMI 18.5 to 25 kg/m(2)), and in normal weight than in underweight (BMI < 18.5 kg/m(2)) for all spirometric variables considered (p trend <or= .022). Although our population studied was predominantly normal weight, we found that higher body mass index may be a modest risk factor for adverse pulmonary effects associated with ozone exposure, especially for women.