The etiology of non-Hodgkin's lymphomas (NHL), including chronic lymphocytic leukemia (CLL), is likely to be related to immune function. In the light of the established immunologic effects of a pregnancy, we decided to examine the risk of NHL and CLL in relationship to full-term pregnancies. Within a nationwide cohort we identified 1,546 women with NHL and 198 women with CLL, all 15 years or older, born 1925-1972. Five age-matched controls were selected for each case patient. Conditional logistic regression was used to estimate the odds ratios after mutual adjustment for number of births and age at first birth. We found a weak, negative association between parity and risk of NHL (p for trend 0.11) and a transient, 10-40% decrease in risk within 5-14 years after the last birth among women with various parity status. The risk of CLL decreased more markedly, and orderly with increasing parity, but the trend was not significant (p = 0.18). Small numbers of cases with CLL prevented more detailed analyses of temporal relationships. Age at first birth appeared unrelated to the risk of both NHL and CLL. We conclude that the immunologic alterations associated with a pregnancy have limited, if any, relevance to the etiology of NHL and CLL; changing reproductive pattern is an unlikely contributor to the marked increase in incidence of NHL seen in many populations.