Because of arthropathic toxicity observed in growing animals the quinolone antibiotics are not recommended for use in children. Recently, magnetic resonance imaging performed in juvenile animals was found to predict ciprofloxacin-induced cartilage damage at the knee joint. We conducted clinical, laboratory, radiologic and magnetic resonance imaging investigations in 13 prepubertal (age range, 6 to 13 years) and 5 postpubertal patients (age range, 14 to 24 years) with cystic fibrosis at the start and the end of a 3-month course of ciprofloxacin (30 mg/kg of body weight/day, administered orally in two equal doses) and at follow-up 4 to 6 months later. Our comprehensive monitoring gave no evidence for arthropathogenicity. Detailed physical skeletal function tests, height velocity values, laboratory studies of bone metabolism and conventional radiographs of both knees revealed no abnormalities. Moreover the serial magnetic resonance images of the left knee demonstrated lack of joint effusion, intact two-layer appearance of the cartilage and unaffected thickness of the articular cartilage measured at five anatomically different points. Our results together with the published data on quinolone use in pediatrics suggest that ciprofloxacin does not cause arthropathy in humans.