This study monitored the effects of seven viral infections on the small bowel. A jejunal mucosa specimen was obtained by peroral biopsy from each patient during the acute phase of illness. Striking gross and histological changes were observed in the jejunal mucosa of six patients with acute viral hepatitis which led to steatorrhea. In contrast, except for the presence of atypical lymphocytes, the jejunal mucosa was normal in six patients with infectious mononucleosis with clinical jaundice. Steatorrhea was not observed in these individuals. Similar jejunal biopsy studies in patients ill with rubella, varicella, and mumps revealed little change in the intestinal mucosa. Giant multinucleated cells were found in the lamina propria of one patient, who developed measles three days after jejunal biopsy. Three of eight patients with adenovirus infections (type 4) showed mild mucosal alterations during the acute phase of their illness.