Groups of young hamsters were exposed to 3, 20, 40, 80, or 160 cercariae. A highly significant correlation was observed between the number of cercariae, worm burdens, and liver and fecal egg counts. The most heavily infected animals were the first to lose weight and die. Hamsters exposed to 20 or more cercariae and harboring a mean of 4.2 or more worm pairs developed significant hepatosplenic disease by 10 weeks after infection as assessed by hepatomegaly, splenomegaly and the development of portal hypertension. Lightly infected animals with single worm pairs did not develop significant disease.