The present article describes the case of a child with cystic fibrosis who presented with acute enterocolitis with severe dehydration subsequently to prolonged antibiotic therapy with oral ciprofloxacin. The associated hypochloraemic metabolic alkalosis led to the diagnosis of salt loss syndrome, which was resolved with i.v. normal saline infusion. The cause of enterocolitis (associated with abdominal pain, hypoproteinemia and increased c-reactive protein) was Clostridium difficile infection, successfully treated with oral vancomycin.