The relative incidence of leukaemia, acute myeloid leukaemia (AML) and central nervous system (CNS) tumours among workers presumably exposed to extremely low frequency (ELF) magnetic fields (MFs) was studied. The study population consisted of all male industrial workers in Finland aged 25 to 64 years during 1971-1980 according to the Population Census in 1970. The occupations were grouped into three exposure categories according to the probability of exposure. The category of "probable" exposure included electrical occupations and the category of "possible" exposure included occupations where electric motors or welding are common. All other occupations were included to the category of "no exposure". Cancer incidence rates in different occupational groups during 1971-1980 were obtained after linking the census records with the national death certificates and the files of the Finnish Cancer Registry. The adjusted relative risks (with 95% confidence limits) in the categories of "probable" and "possible" exposure were for all leukaemia 1.9 (1.0-3.5) and 1.4 (1.1-1.8), for AML 1.5 (0.5-4.7) and 1.4 (0.9-2.1), and for CNS tumours 1.3 (0.7-2.3) and 1.3 (1.0-1.6), respectively. The results are concordant with earlier studies suggesting elevated risk among workers exposed to ELF magnetic fields.