Groups of mice infected with Schistosoma japonicum were killed 7, 10 or 15 weeks after infection. The size of hepatic granulomas and degree of hepatic fibrosis in individual mice were examined in relation to the number of worm pairs present using regression analysis. Mice with heavy S. japonicum infections had smaller circumoval granulomas in their livers than did mice with lighter infections. Hepatic fibrosis per unit infection, i.e., per worm pair or per egg, also decreased as the intensity of infection increased. These trends were similar in 8 strains of mice examined. No clear trends were found in rabbits or monkeys infected with S. japonicum. Hepatic fibrosis in S. haematobium-infected mice decreased with increasing intensity of infection, although this trend was significant only in infections of 1 year duration. C57BL/6 mice infected with S. mansoni often showed trends similar to those in S. japonicum-infected mice; however the findings in C3H and ICR mice infected with S. mansoni were more variable. Most of the data for the present report were from animals also described in previous publications. The relevance of these findings to schistosome infections in humans is unknown but the results are clearly pertinent to the analysis of hepatic pathology in experimental infections. Overall egg production by the worms was not affected and the number of eggs per worm pair recovered from the tissues was not generally influenced by the number of worm pairs present.