A right iliac fossa mass may be a difficult diagnostic problem in a pa
tient with cystic fibrosis. We present a patient with such a mass who
was thought to have a nonobstructing intussusception on clinical and r
adiological grounds. However, at laparotomy she was found to have a pa
thology not previously described in cystic fibrosis. She had a partial
diaphragm almost blocking the lumen to her appendix. The differential
diagnosis of a right iliac fossa mass is considered and the cause of
her pathology discussed.