Register and census data for complete cohorts of Norwegian men and women born between 1935 and 1974 were used to examine the relationship between reproductive factors and the incidence of Hodgkin's disease (HD). Among 1.3 million men and 1.3 million women under observation, 695 male and 441 female cases of HD were diagnosed during the period of follow-up. Our hazard model estimates showed that women, at a given age and in a given birth cohort, have an HD incidence inversely related to current parity. A clear relationship was found only for the nodular sclerosis subtype. In men, the risk of HD development was higher than that in childless women, and there was no parity effect. The lower HD incidence among high-parity women could not be ascribed to their lower social status. Presumably, there is a still unidentified protective factor associated with the biology of childbearing, the effect of which possibly wears off with increasing length of time since childbirth. In addition, there are indications of a net effect of age at entry into motherhood, which may explain part of the estimated parity effect.