In 50 patients with prostatic hyperplasia renal function was examined in regard to 1. Incidence and type of impairment; 2. Correlations between the measures of prostatic hyperplasia and the degree of impairment of renal function; 3. The role of urinary infection in impairment of renal function. In this series tubular function proved to be more impaired than the glomerular filtration rate. No correlation could be found between the measures used to express the degree of prostatic hyperplasia and the impairment of renal function. The reasons for this are discussed. The coincidental urinary infection proved to exert a significantly detrimental effect on renal function. The importance of urinary infection in planning therapy in prostatic hyperplasia is emphasized