We present a case of collagenous colitis with evidence of terminal ileal involvement. The patient suffered from chronic watery diarrhea of three months' duration, and colonoscopy revealed a normal endoscopic appearance. Histologic examination of biopsies revealed a broad subepithelial band of collagen, with similar histologic findings in the terminal ileum. Additionally, the patient exhibited abnormalities of d-xylose and vitamin B12 absorption, although there were no clinical signs of malabsorption. Biopsy of the proximal small intestine was normal. The significance of these findings for the definition of the clinicopathologic entity of collagenous colitis and its pathogenesis are discussed.