In a retrospective follow-up we compared the incidence of malignancies in patients with rheumatoid arthritis treated with cyclophosphamide with that in another group of patients with rheumatoid arthritis and also with the incidence of malignancies in the general population. Among 81 patients treated with cyclophosphamide in the past decade 15 malignancies occurred. This was 4.1 times the expected number obtained from a closely matched control group of patients with rheumatoid arthritis not treated with cytotoxic drugs (95% confidence interval 1.5 to 19.0), and 3.7 times the expected number calculated from general population rates (95% confidence interval 2.1 to 5.9). The increase in haematological and lymphoreticular malignancies was specially notable. The data also indicate that the development of malignancies after the start of cyclophosphamide therapy necessitates a certain induction time and that it is to some extent dose-dependent.