The contribution to diagnostic understanding and therapeutic decision making by computed body tomography (CT) was measured in 623 patients using prospective and retrospective inquiry of referring physicians. CT improved diagnostic understanding in 52% of patients, reassured the physician about previously planned therapy in 43%, improved precision of previously planned treatment in 23%, and contributed to a change in therapy in 14%. There was a significant increase over time in reports of improved diagnostic understanding which was probably related to increasing clinical experience with the technique. However, this improvement in diagnostic understanding did not correspondingly increase the frequency of beneficially altered therapy. CT contributed to a substantial decrease in preplanned invasive imaging tests and a decision not to perform surgery in 19% of patients in whom an operation had been previously planned. The value of a CT examination to patient care varied considerably with the clinical problem; the most consistent contributions were observed in patients with diseases suspected in the mediastinum and retroperitoneum.