Acetaminophen decreases glutathione levels in the lung, which may predispose to oxidative injury and bronchospasm. Acetaminophen use has been associated with asthma in cross-sectional studies and a birth cohort. We hypothesized that acetaminophen use would be associated with newly diagnosed adult-onset asthma in the Nurses' Health Study, a prospective cohort study of 121,700 women. Participants were first asked about frequency of acetaminophen use in 1990. Cases with asthma were defined as those with a new physician diagnosis of asthma between 1990 and 1996 plus reiteration of the diagnosis and controller medication use. Proportional hazard models included age, race, socioeconomic status, body mass index, smoking, other analgesic use, and postmenopausal hormone use. During 352,719 person-years of follow-up, 346 participants reported a new physician diagnosis of asthma meeting diagnostic criteria. Increasing frequency of acetaminophen use was positively associated with newly diagnosed asthma (p for trend = 0.006). The multivariate rate ratio for asthma for participants who received acetaminophen for more than 14 days per month was 1.63 (95% confidence interval, 1.11-2.39) compared with nonusers. It would be premature to recommend acetaminophen avoidance for patients with asthma, but further research on pulmonary responses to acetaminophen is necessary to confirm or refute these findings and to identify subgroups whose asthma may be modified by acetaminophen.