The relationship between reproductive factors and risk of lymphoid neoplasms was investigated in a hospital-based case-control study conducted in northern Italy on women with histologically confirmed incident Hodgkin's disease (HD) (n = 68), non-Hodgkin's lymphomas (NHL) (n = 180) and multiple myelomas (MM) (n = 71), and 448 controls admitted to hospitals, for acute, non-neoplastic, non-immunological and non-gynecological conditions. The odds ratios (OR) of HD were 0.6 for > or = 3 pregnancies compared to nulligravidae, and 0.5 for > or = 1 total (spontaneous and induced) abortions compared to women reporting no abortions. Compared to nulliparae, the OR of HD was 0.9 in parae and 0.3 in those with first birth when aged < 20 years. The OR of NHL and MM in relation to number of pregnancies, abortions and births, age at first birth and time since last birth were close to unity. Results were similar for the relation between reproductive factors and HD in women younger than 50 years. The OR of NHL was above unity (OR 2.2, 95% CI 1.0 to 4.9) for women aged < 50 years reporting one or more pregnancies as compared to nulliparae, and for women reporting the last birth since less than 10 years (OR 2.9, 95% CI 1.1 to 7.4). Early events in pregnancy, including changes in immunological status, rather than exposure to female sex hormones are likely mechanisms for the protection of pregnancies and abortions on the risk of HD.