A prospective case-control study of the aetiologic factors involved in the production of lymphoid malignancies has been conducted within a defined geographical area covering six health districts in the Yorkshire Region. Among the aspects investigated were past medical events, occupations and certain social factors. A number of possible causal relationships have been identified including Jewish religion, past solvent exposure and ingestion of amphetamines, although the latter did not achieve statistical significance in this study. In addition several new associations have been identified, most notably with the occurrence of adult eczema/dermatitis and with treatment by radiation or steroids. The feasibility of conducting such a broadly based epidemiological investigation has been established.