Cyclosporine therapy is complicated by nephrotoxicity that is not predicted by drug levels. In this study serial trough blood samples were obtained from 11 allogeneic marrow transplant recipients after initiation of intravenous cyclosporine 2 mg/kg every 12 hours for a period extending 4 weeks after transplantation. Renal dysfunction, assessed by an increase in serum creatinine levels to twice baseline values or when greater than 175 mumol/L, was found in four patients. No associations between renal dysfunction and cyclosporine levels in whole blood, total plasma, or lipoprotein fractions were found. The ratios of maximum and mean high-density low-density lipoprotein cyclosporine concentrations were greatest in patients with renal dysfunction (p less than 0.001). The data suggest therapeutic drug monitoring of cyclosporine in various biologic fluids does not predict onset of drug-associated renal dysfunction. However, the relative role of high-density to low-density lipoprotein transport of cyclosporine may provide an index of renal functional changes associated with the agent.