STUDY OBJECTIVE: Experimental data suggest sleep deprivation may impair host immunity. We sought to assess the effect of poor sleep on pneumonia risk. DESIGN: Prospective, observational cohort study. PARTICIPANTS: 56,953 female nurses (ages 37 to 57 years old) participating in the Nurses' Health Study II cohort free of cancer, cardiovascular disease, diabetes, and asthma with no prior history of pneumonia. MEASUREMENTS AND RESULTS: At baseline, participants reported their average sleep duration and whether this quantity was adequate for them. Questionnaires ascertaining a new pneumonia diagnosis were mailed every 2 years. Cases required physician diagnosis and chest radiograph confirmation. Cox proportional hazards models were used to assess the relative risk for incident pneumonia over 4 years. Over 217,500 person-years, 977 cases of pneumonia were identified. Relative to 8-h sleepers, both short and long sleep durations were associated with elevated pneumonia risk. The age-adjusted relative risk for pneumonia was 1.70 (95% CI 1.30-2.23) in those sleeping ≤ 5 h and 1.49 (95% CI 1.12-1.98) in those sleeping ≥ 9 h. After adjusting for potential confounders, the relative risks were 1.39 (95% CI: 1.06-1.82) in those sleeping ≤ 5 h and 1.38 (95% CI 1.04-1.84) in those sleeping ≥ 9 h. Perceived inadequate sleep was also associated with pneumonia with a relative risk of 1.50 (95% CI 1.29-1.74) in multivariate models. CONCLUSIONS: Both reduced and prolonged habitual sleep durations are associated with increased risk of pneumonia. Further research is needed to understand how sleep habits can influence immunity.