A nonsteroid antiinflammatory agent (Ibuprofen) was used in a controlled randomized study to determine its ability to replace steroids in the prophylaxis of cadaveric kidney rejection. Thirty-three cadaver kidney recipients were randomly assigned either to a control group (16 patients) receiving azathioprine, high doses of prednisolone, and antithymocyte globulin (ATGAM) for three months, or to an experimental group (17 patients) receiving azathioprine and ATGAM according to the same protocol, ibuprofen instead of steroids. The frequency of rejection was higher in the experimental group (2.18 episodes per patient) than in the control group (1.44 episodes per patient). Nevertheless, in the experimental group 5 patients had no early rejection episode, 60% of early rejections were totally reversible without steroids, and 3 patients never received steroids at all during the first year and had normal renal function and biopsies. Steroids had to be introduced in the treatment of 14 patients, but after an average period of 32.5 days after surgery OKT3+ cell level was higher in the experimental group than in the control group, but similar to the OKT3+ cell level of patients receiving conventional therapy without ATGAM. Whatever the type of treatment, an increase in the OKT4+/OKT8+ ratio was associated in most cases with increased serum creatinine values. Conversely, a decreased OKT4+/OKT8+ ratio associated with renal failure was found in cases showing biological evidence of cytomegalovirus infection.