The incidence of multiple malignant neoplasms in 55,279 patients with rheumatoid arthritis (RA) or allied disorders was studied by linking two separate nationwide data registers covering the whole Finnish population. The linking of the Social Insurance Institution's Population Register and the Finnish Cancer Registry resulted in 2106 cancer patients in whom both cancer (diagnosed in 1967-1978) and rheumatoid arthritis or allied disorders (diagnosed before cancer) were diagnosed. In 1974, the allied disorders (ankylosing spondylitis or systemic connective tissue diseases) accounted for 3.9% of all the diagnoses in the Social Insurance Institution's Population Register. The incidence of multiple neoplasms was slightly but not significantly higher in cancer patients with RA or allied disorders than in the general population or in all cancer patients taken together. The risk of leukemia was four times higher in the rheumatoid or allied disease cancer patients than in all the cancer patients. The results are consistent with previous results showing that the leukemia incidence in RA patients is higher than expected.