Appendico-cutaneous fistulae are rare. We describe a 12-year-old girl with cystic fibrosis (CF) and a chronically draining sinus in the right iliac fossa, found to be a primary appendico-cutaneous fistula. Misdiagnosis of right iliac fossa pain in patients with CF and the preponderance of complicated disease frequently lead to manifestations of appendicitis rarely seen in usual clinical practice.