Elimination half-life times, plasma concentrations, and minimal cumulation factors are being determined resp. calculated for trimethoprime and sulfametrole (free as well as total amounts)--both combination partners of Lidaprim--in healthy volunteers and in patients with impaired renal function. There is a strong influence of renal function on the cumulation of the metabolized part of the sulfonamide, a weaker one on trimethoprime, and almost none on the free sulfonamide. On account of the sulfonamide metabolite cumulation, a reduction by half of the standard dose of Lidaprim is recommended if the creatinine clearance is below 20-30 ml/min.