In a study with 30 randomized patients the pharmacokinetics, efficacy and side effects of Sisomicin were determined. Half of the patients received one single intramuscular injection of 75 or 100mg (1.4 mg/kg) Sisomicin/day. The other cases were injected 50 or 75mg (1.9 or 2.1 mg/kg) b.i.d. The serum concentrations determined on the first and last days of treatment 1 hour after the application ranged between 4 and 5.3 mug/ml. After 6 hours the values decreased to 0.7-0.8 mug/ml. The elimination rate within 24 h amounted to 63-77%. Depending on the dosage group, the mean urinary concentrations/24 h amounted to 46-51 mug/ml or 63 to 92 mug/ml, respectively. The highest concentration measured during the first 8 hours amounted to 129 mug/ml after an administration of 1.4 mg/kg. In 18 of 29 cases a "cure" of urologic infections, most of which had been treated previously without success, could be achieved. Furthermore we observed 5 relapses, 3 failures and 3 reinfections, but no superinfection. The local and general tolerance of Sisomicin was good. 5 patients showed toxic reactions of the vestibular organ in the form of dizziness. In 1 case there was an excretion of granulated casts.