Acute impairment of renal function caused by cyclosporin A can be hard to differentiate from acute rejection. Therefore, kidney function after cadaveric allograft transplantation was repeatedly determined by renography in 42 patients receiving either high dose cyclosporin A (32 patients) or azathioprine and prednisone (10 patients) until a graft biopsy showed either acute rejection or no rejection within the first 5 postoperative weeks. The graft function as judged from the renograms was significantly poorer when cyclosporin A was used than when azathioprine and prednisone were the immunosuppressants. In the azathioprine and prednisone group a biopsy showing acute rejection was always preceded by a deterioration in the renogram. In cyclosporin A treated patients a graft biopsy following an early deterioration in the renogram showed acute rejection in only 56% of the biopsies. It was not possible to identify a time course or a function level of the renogram that could predict rejection in these patients. It is concluded that graft biopsies should be used liberally to diagnose rejection during cyclosporin A treatment if surgical complications after transplantation have been ruled out. Radionuclide studies may offer an invaluable aid in determining a nonnephrotoxic initial dose of the drug.