A Royal Naval submariner recently developed Chronic Myeloid Leukaemia after 13 years in the submarine service. The Institute of Naval Medicine therefore reviewed the submariner's occupational exposure history to benzene with a view to determining whether this had been a causative agent in the development of the disease. Past atmosphere patrol reports taken from the submarines on which the submariner served were scrutinised to determine the maximum and median weighted levels of benzene to which he was exposed over the 13 year period. These were 830 micrograms m/m3 and 189 micrograms m/m3 respectively. To determine what increased risk, if any, is associated with the submariner's exposure to benzene at these levels it was necessary to compare the data with the results obtained from epidemiological studies of workers exposed to benzene in various industries. Industrial epidemiological studies have proved statistically that benzene induces leukaemia above occupational exposure levels of 16 mg/m3 (5ppm). Below this level there is no statistical evidence that benzene is carcinogenic and estimates of increased risk have therefore necessitated the use of mathematical modelling. The estimated range of increased leukaemia cases due to benzene exposure is 0.5-6.6 additional cases per 1000 workers exposed over a working lifetime of 40 years (40ppm-years). The increased risk to the submariner using the median submarine benzene levels was calculated to be 0.02-0.3 additional cases (of all types of leukaemia) per 1000 submariners. The increased risk of developing the rare Chronic Myeloid Leukaemia from benzene exposure in submarines is considered to be extremely small.