The evidence regarding reproductive events as risk factors for rheumatoid arthritis (RA) is not conclusive. In the present study a population-based cohort consisting of 63,090 women were followed from 1961 to 1989. Detailed data on reproductive factors were collected through personal interviews in the period 1956-9. The endpoint used was mention of RA on the death certificate. Mortality rate ratios were estimated by Poisson regression, controlling for various demographic variables. During 1,485,400 person-yr of observation, a total of 355 cases with RA mentioned on the death certificate were identified. Total time of lactation was associated with a decreased mortality of RA, with an approximate dose-response relationship. The results did not support a role of parity, age at first and last birth, or age at menarche and menopause in the development of RA. This protective effect of lactation on the development of RA has not previously been described, and since a definite biological explanation is lacking, the association should be confirmed in other populations.