To investigate the possible role of previous medical history and previous medications as risk factors for hairy cell leukaemia (HCL) we performed a population based case-control study on 121 male HCL patients and 484 controls. The data were collected through a self-administered mailed questionnaire. Elevated odds ratios (OR) were found for a history of appendicitis [OR 1.9; 95% confidence interval (CI) 0.9-4.2] and pneumonia (OR 2.9; CI 0.9-9.6). We found a reduced risk for HCL associated with a history of myocardial infarction (OR 0.3; CI 0.4-2.5), hypertension (OR 0.6; CI 0.3-1.2) and thromboembolic disease (OR 0.6; CI 0.1-2.7). Reduced OR was also associated to a history of diabetes mellitus (OR 0.6; CI 0.1-2.9) and a diagnosis of hyperlipidemia (OR 0.8; CI 0.2-3.6). HCL is an indolent disease with a clinical course of many years and it can not be excluded that the disease leads to metabolic changes, resulting in a changed risk for these diagnoses. When the role of previous medications were investigated, increased OR was found for NSAID (OR 3.4; CI 1.1-10.2). Decreased OR was found for the anti-coagulative agent warfarin (OR 0.4; CI 0.1-1.5). A history of a previous malignancy preceeding the diagnosis of HCL as reported to the Swedish Cancer Registry yielded an increased OR of 3.2 (CI 1.2-8.5). All results must be interpreted with caution, as there is a possibility of misclassification. Medications is difficults to remember, particularly several years after consumption. As many comparisons were made, there is always a possibility of correlations occuring by chance.