Cyclosporine (CsA), a powerful immunosuppressive agent that increases graft survival in renal transplant recipients, is often nephrotoxic. The clinical distinction between acute rejection and CsA nephrotoxicity (NT) is a common challenge in the management of these patients. To seek a histologic distinction between acute rejection and CsA-NT, we reviewed the renal biopsies performed prior to initiation of therapy for rejection or nephrotoxicity in two groups of patients. Group 1 (ten patients) had criteria consistent with acute rejection and responded to steroid pulse therapy. Group 2 (15 patients) was treated for CsA-NT and responded to a decrease in the dose of CsA. We conclude that CsA-NT has no specific histologic features. A prominent interstitial mononuclear cell infiltrate as well as tubulitis are features of acute cellular rejection. These findings do not exclude the possibility that rejection and CsA-NT can co-exist in the same patient.