The Cystic Fibrosis Clinic at the Royal Belfast Hospital for Sick Children has treated 31 children with ciprofloxacin, for serious pseudomonas infection in cystic fibrosis, and carefully monitored the safety and acceptability of the drug. Initially, eight very ill children were treated on a named-patient basis, with an encouraging clinical response and few adverse effects. Children aged 10-18 years were included in a study of four consecutive exacerbations of respiratory disease, comparing (i) oral ciprofloxacin in each episode with (ii) ciprofloxacin alternating with intravenous azlocillin and tobramycin. Other children with cystic fibrosis were subsequently treated with ciprofloxacin, as the need arose. In all the groups very few adverse reactions were found; in particular only one child developed arthralgia. A total of 202 children in the UK have been treated with ciprofloxacin on a named-patient basis, and their clinicians have reported 46 adverse events that may have been drug-related. Overall ciprofloxacin appears to be safe and effective in children but concern about the possible occurrence of arthropathy remains and long term follow-up of these children may be necessary.