Fifty in-patients suffering from chronic urinary tract infections with underlying urological diseases received the oral Carbenicillin preparation Indanyl Carbenicillin (short term Carindacillin) for 10 days in a dosage of 1 g q.i.d. The therapy results were objectified by blood and urine tests performed prior to and after treatment. Indanyl Carbenicillin led to a sterile urine in nearly 60% of the patients; in about 85% of the cases the primary organism was eliminated. The drug was particularly active against difficult organisms such as Proteus, Klebsiella and Pseudomonas. The results obtained in this study are somewhat surprising as the dosage used produces bactericidal levels of the active substance only in the cavity system of the efferent urinary tract but not in the tissue. No effect on blood picture or liver function was observed. Side effects occurred in 8 patients, they disappeared with continued therapy or after discontinuation of the drug. Considering the good efficacy observed in this study and because of the possibility of oral administration, Indanyl Carbenicillin appears to be suitable also for out-patient treatment of chronic urinary tract infections due to difficult urological germs.