In a study of the distribution and severity of colonic lesions in patients with shigellosis, colonoscopy was performed for 33 men with this disease. All 33 patients had inflammatory lesions in the rectosigmoid area; in 18 (55%) the lesions extended to the splenic flexure, in 14 (42%) the disease extended to the distal transverse colon, in nine (27%) the area of involvement included the proximal transverse colon, and in five (15%) pancolitis was evident. In most patients lesions were continuous and diffuse, with the intensity of inflammation decreasing in a proximal direction. Biopsied samples from proximal lesions usually showed less severe inflammation than did those from more distal lesions. Aphthoid erosions, which have not previously been described in shigellosis, were observed in five patients. Proximal colitis was associated with diarrhea of four or more days' duration (P less than .01, Fisher's exact test). These findings indicate that the rectosigmoid is the most frequently and most severely affected area of the colon in shigellosis and suggest that during the course of shigella infection, colonic lesions extend in a proximal direction.