The risk of cancer among multiple sclerosis (MS) patients was evaluated emphasising cancers with a potentially infectious aetiology. Cancer incidence was estimated among incident MS patients in 1964-1993 (n = 1,597) in Finland. The cohort was followed up for cancer incidence through the Finnish Cancer Registry until 1999. A total of 85 cancer cases were diagnosed showing a standardised incidence ratio (SIR) of 1.0 (95% CI 0.8-1.2) for all cancers. The risk (SIR) of haematological tumours was 1.1 and that of central nervous system (CNS) tumours 1.3. The small excess risk of haematological malignancies is consistent with infectious aetiology, whereas the association between MS and CNS tumours may be due to misclassification.