The absorption of lactulose and mannitol in 20 patients with Crohn's disease limited to the ileum or colon was studied, and lactulose/mannitol excretion ratios were calculated. The results were compared to those from 16 normal controls and 6 patients with ulcerative colitis. The 13 patients with ileal Crohn's disease had significantly higher lactulose/mannitol excretion ratios than the controls (p less than 0.01) or ulcerative colitics (p less than 0.01). Similarly, the 7 patients with Crohn's disease limited to the colon had significantly higher excretion ratios than the controls (p less than 0.01) or ulcerative colitics (p less than 0.01). The results provide support for the concept that Crohn's disease may be more extensive than is apparent macroscopically.