The cancer risk was studied by comparison of 305 rheumatoid arthritis (RA) patients exposed to Proresid during a mean time of 22 months and 305 RA patients exposed to sodium aurothiomalate during a mean time of 19 months with the regional cancer register. The mean observation time was 6.9 years (2,117 person-years) for the Proresid-treated and 7.5 years (2,293 person-years) for the gold-treated patients. No increased risk of total malignancies was observed for either group. However, looking at separate tumours, an increased risk of lymphoma and leukemia was found although only significant in the gold-treated group. It was not correlated to dosage or duration of either therapy. The increased risk is consistent with earlier reports of an increased risk of hematopoietic malignancies in RA patients. Marginal over and underreporting, particularly of hematopoietic malignancies, were observed, mainly due to clinicians' failure to report and to recall false reports.