Plasma and urine concentrations of creatinine, sulphamethoxazole and its metaboliteN4-acetylsulphamethoxazole were measured in patients with varying degrees of kidney impairment. The plasma elimination half-life of sulphamethoxazole is not influenced by the kidney function, while the half-life of the metabolite is fully dependent on the kidney function. The renal clearances of the metabolite and that of the parent compound under alkaline urine conditions are linearly related to the creatinine clearance. Kidney impairment affects only the renal clearance of the metabolite, whereas sulphamethoxazole is metabolized (acetylated) independently of kidney function.