A 14-day ascending dose course of sirolimus (rapamycin, RAPA) was administered to quiescent renal transplant patients receiving a double-drug cyclosporine (CsA)/corticosteroid regimen in a double-blinded randomized study. Oral sirolimus or placebo was delivered twice daily in divided doses for 13 days and a final dose was administered on the morning of study day 14. In addition, patients in the sirolimus- and placebo-treated groups were compared with a demographically matched, concurrently treated control cohort of 30 patients who received the same concentration-controlled CsA/corticosteroid regimen. The study cohort was partitioned into four sirolimus dose level groups: placebo (0 mg/m2/day, N = 10), low dose (1 to 3 mg/m2/day, N = 9), medium dose (5 to 6 mg/m2/day, N = 9), and high dose (7 to 13 mg/m2/day, N = 12). the primary side effect of sirolimus was a reversible decrease in platelet (PLT) and white blood cell (WBC) counts. Cholesterol values increased statistically significantly in the sirolimus-treated patients when compared with those of the placebo patients, but not when compared with those of the control group patients. There were no statistically significant differences in the steady-state average concentrations of CsA among sirolimus dose groups (including placebo). No differences were observed between the pre- and post-sirolimus treatment values of systolic and diastolic blood pressure values, glomerular filtration rates (GFR), serum creatinine values (SCr), and serum glutamic oxaloacetic transaminase (SGOT), serum glutamic pyruvic transaminase (SGPT) or triglyceride levels. Because the principal side effects of sirolimus are distinct from the principal nephrotoxic properties of CsA, this drug combination may display potent immunosuppression without exacerbated toxicity.