An approximation of the entire population (83%) of the village of Lower Nduu, Machakos Kenya, was examined in a cross-sectional study of the prevalence and intensity of schistosomiasis mansoni correlated with morbidity as determined by standard medical examination. The 416 individuals in 90 households studied had a male:female ratio of 44:56 and an age structure in which more than 50% were below age 20. Malaria was of very low endemicity in the area as were intestinal helminths and Schistosoma haematobium. S. mansoni infection as determined by quantitative Kato thick smears had an overall prevalence of 82% with a peak of 98% in the 10 to 19 age group and a decline to about 70% in the older population. With respect to intensity, 12% were uninfected, 28% had light (10–100 eggs/g), 21% moderate (101–400), and 33% heavy (>401) infections; 16% had egg counts over 1,000/g. Peak intensity occurred in females at ages 10 to 14 years (1,026 eggs/g) and in males at age 20 to 24 years (1,019 eggs/g). A history of weakness or inability to work occurred in only a small proportion of the population and was not correlated with intensity of infection. Abdominal pain was significantly more prevalent in the most heavily infected individuals (>1,000 eggs/g). Hepatomegaly occurred more frequently in those more heavily infected (>400 eggs/g) and splenomegaly, which was not observed in the uninfected population, occurred in 7% of those with egg counts greater than 400/g. Twelve individuals, or almost 3% of the population studied, had hepatosplenomegaly: most of them were under 21 years of age, all of them were infected with S. mansoni, and 58% had high egg counts. Liver function tests were mostly within normal limits. Two of the patients had esophageal varices on barium swallow, but these and 7 others studied by splenoportography had patent portal veins.