Reasons for higher incidence of lymphoid neoplasms among men than women are unknown. Because female sex hormones have immunomodulatory effects, reproductive factors and exogenous hormone use may affect risk for lymphoid malignancies. Previous epidemiologic studies on this topic have yielded conflicting results. Within the National Institutes of Health-AARP Diet and Health Study cohort, we prospectively analyzed detailed, questionnaire-derived information on menstrual and reproductive factors and use of oral contraceptives and menopausal hormone therapy among 134,074 US women. Using multivariable proportional hazards regression models, we estimated relative risks (RRs) for 85 plasma cell neoplasms and 417 non-Hodgkin lymphomas (NHLs) identified during follow-up from 1996 to 2002. We observed no statistically significant associations between plasma cell neoplasms, NHL, or the 3 most common NHL subtypes and age at menarche, parity, age at first birth, oral contraceptive use or menopausal status at baseline. For menopausal hormone therapy use, overall associations between NHL and unopposed estrogen and estrogen plus progestin were null, with the potential exception of an inverse association (RR = 0.49, 95% CI, 0.25-0.96) between use of unopposed estrogen and diffuse large B-cell lymphoma (DLBCL), the most common NHL subtype, among women with a hysterectomy. These data do not support an important role for reproductive factors or exogenous hormones in modulating lymphomagenesis.