Crohn's disease (CD) arising in children with cystic fibrosis (CF) is well recognized. Indeed, reports suggest that CD is significantly more common in patients with CF than in the general population. Giant inflammatory polyposis is a rare manifestation of idiopathic inflammatory bowel disease and may complicate both ulcerative colitis and CD. Giant inflammatory polyposis has not been specifically reported in patients with coexistent CF and CD. Herein, we report the occurrence of giant inflammatory polyposis in 2 boys attending a tertiary care hospital, with an established diagnosis of CF who subsequently developed CD. Both boys required surgical treatment for CD. In addition to classical features of CD, both colonic resection specimens showed giant inflammatory polyposis. The appearances were modified by the presence of a layer of thick mucus. It is suggested that the coexistence of CF in patients with CD may predispose to the development of giant inflammatory polyposis. In addition to contributing to their development, it also appears that there is a propensity for CF to alter the morphological appearance of giant inflammatory polyposis. This may lead to diagnostic confusion when examining endoscopic biopsies.