Three cases of intestinal schistosomiasis acquired in northern Laos are reported. All three patients swam regularly for many years in the Mekong River near the cities of Vientiane and Luang Prabang. Morphology of the eggs found on rectal biopsy indicated infection with Schistosoma mekongi. One patient had severe portal hypertension with bleeding esophageal varices requiring a splenorenal shunt; studies of her liver biopsy showed S. mekongi eggs and periportal fibrosis. A second patient had abnormal liver function tests, and a third patient was asymptomatic. In two of the three cases, no schistosome eggs were found on stool examination, and the diagnosis was made by rectal biopsy. The implications of this new focus of infection are discussed.