The pharmacokinetics of a single intravenous dose of 30 mg of cefoxitin/kg given to normal subjects and to patients with various degrees of chronic renal insufficiency were studied. The serum half-life of cefoxitin was correlated inversely with the rate of creatinine clearance, and the half-life increased exponentially when the creatinine clearance rate fell below 30 ml/min per 1.73 m2. The elimination of cefoxitin into urine was rapid and extensive in patients whose creatinine clearance was greater than 30 ml/min per 1.73 m2. In patients with more severe degrees of renal impairment, renal clearance of drug was diminished markedly (less than 20% of the administered dose was excreted in 4 hr as compared with greater than 60% in the former group). The clearance of cefoxitin can be increased fivefold by hemodialysis in patients with end-stage renal disease.