In spite of introducing the new derivatives of fluoroquinolones into treatment in late 70's, the application this kind of chemotherapeutics in children is still controversial. The aim of our study was the evaluation of treatment efficacy and adverse effects associated with the application of ciprofloxacin in premature infants within first months of life. The investigations were performed on 36 premature infants delivered between 25-35 gestational age with birth weight varied from 750-2050 grams, hospitalized in 1993-1999 in Emergency Unit and Department of Pediatric Propedeutics of Institute of Pediatrics of the Medical University of Łódź. At the beginning of the therapy the age of our patients varied from 10 to 202 days. Ciprofloxacin 13.8 mg/kg/day in two or three divided doses was administered. The time of treatment varied from 3 to 20 days. Sepsis was the most frequent cause of application of this drug. The following treatment of ciprofloxacin was performed on three children within one, two and three months. The efficacy was assessed as good in 66% cases of treatment course. The following adverse effects were observed during and after the treatment: thrombocytopenia (5 cases), elevated transaminases (3 cases), hyperbilirubinemia (3 cases), the elevation of creatinine concentration varied from 0.2 mg% to 0.6 and from 0.1 to 0.95 mg% in two patients. Moreover one child developed femoral osteitis. Longitudinal studies concerning physical development and health state of these patients are being done.