Plasma and urine levels of 12 healthy subjects and 30 patients with renal insufficiency of different degrees were examined after oral administration of four 250 mg capsules azithromycin (total daily dose 1,000 mg). The concentrations were determined by cup plate method. The pharmacokinetic parameters were determined model-dependent and noncompartmentally. Neither the area under the plasma concentration curve nor the distribution volume in steady state (16 l/kg body weight) nor the maximal plasma concentration were significantly affected by renal insufficiency. Thus the dosage regimen of azithromycin in renal impairment may (and should) be the same as in patients with normal renal function. The nonrenal clearance is not affected by renal insufficiency, but the concentration of the substance in the tubular lumen (the "tubular load") may be increased.