The pharmacokinetics of cefotetan following a 0.5 g intravenous dose were studied in 15 patients with varying degrees of renal failure. In patients with glomerular filtration rates (GFR) greater than 50 ml/min the serum half-life was between 3 and 5 h. The half-life increased to 15 h as the GFR fell to 10 ml/min. When the GFR was less than 10 ml/min the serum clearance of cefotetan was very variable, the half-life being between 15 and 50 h. Cefotetan was eliminated almost entirely by the kidneys. It is suggested that cefotetan 0.5 g 12-hourly is appropriate for patients with GFRs greater than 20 ml/min. For patients with GFRs between 10 and 20 ml/min 0.5 g every 24 h should avoid gross accumulation.