Cambridge Health Authority accepted the gift of a whole body computed tomography (CT) scanner and radiotherapy planning system for Addenbrooke's Hospital (AH) from a local charity in 1981. The charity was to provide funds for the maintenance and all running costs for the first 5 years. Acceptance of the gift was conditional on an evaluation of the use of body CT before any decision about subsequent funding was made. The results of this evaluation are presented here. A detailed assessment fo 150 consecutive CT studies was performed together with an audit of CT use. The costs of running the CT unit and an indirect measurement of the benefits brought about by body CT were evaluated. In 86% of patients the CT findings were considered likely to have a great contribution to clinical management; in less than 10% was little or no benefit likely. In most patients (68%) CT was considered to have been more useful than any alternative test(s) would have been. The total cost of CT during the financial year 1983-84 was 219,163 pounds and the average cost per CT study was 73 pounds. The assessment indicated annual benefits fo 673,385 pounds including the saving of 3,880 inpatient bed days, 760 outpatient visits and alternative tests costing 290,684 pounds p.a. Some corroborative evidence that these benefits have occurred is presented. This study suggests a cost benefit ratio of 1:3.07.