Little information is available about the clearance of pentoxifylline and its metabolites in renal failure. Consequently, when a dialysis patient required this drug, we started at a low dose and gradually increased the dosage while monitoring the patient for signs or symptoms of toxicity and following plasma concentrations of parent drug and its metabolites. Our patient appeared to develop evidence of drug toxicity after about six days on two-thirds of the usual recommended adult dose of the drug. The pentoxifylline half-life and apparent half-life of metabolite I were both substantially prolonged as compared with data from healthy volunteers. Both accumulated rapidly in plasma. Concentrations of metabolites IV and V were also very high.