Conversion of stable renal transplant patients from Sandimmune to Neoral may pose a risk of short-term nephrotoxicity. Serial serum creatinine concentrations were measured in 141 kidney and simultaneous kidney-pancreas transplant recipients converted from Sandimmune to Neoral on a 1:1 dosing basis and followed for up to 11 months. Following conversion, cyclosporine dose was reduced in 74% of patients with a mean dose reduction of 22.6% for the entire cohort. Serum creatinine concentrations transiently increased to more than 30% above baseline in 48% of patients and remained more than 30% above baseline in 16% of patients. Multivariate analyses suggested that acute rejection, maintenance steroid therapy, young age, Sandimmune dose prior to conversion, and an increment in trough cyclosporine levels after conversion were positive predictors of renal insufficiency following conversion. Lower doses of Neoral should be considered at the time of conversion to minimize nephrotoxicity.