The relationships between S. haematobium, hookworm, malaria, hemoglobin level, splenomegaly, and hepatomegaly before and 8 months after treatment with a single dose of metrifonate or praziquantel were studied in Kenyan primary schoolchildren in an area where anemia, S. haematobium, and hookworm are common and malaria is holoendemic. Children with light to moderate S. haematobium infection were examined (Exam 1), assigned at random to groups receiving placebo (PL, n = 104), metrifonate (MT, n = 103, dose 10 mg/kg body weight) or praziquantel (PR, n = 105, dose 40 mg/kg body weight), treated, and examined 8 months later (Exam 2). At Exam 2, 62% of the MT group still passed S. haematobium eggs vs. 13% in the PR group. Egg reduction rates were substantial in both groups, but greater in the PR group; geometric mean egg counts in both groups were very low. Prevalence and intensity in the PL group had not changed between exams. Hookworm egg counts were significantly reduced in the MT group (59% egg reduction rate); malarial infection had increased in all 3 groups, presumably due to the long rainy season between exams. Hookworm egg count was the most significant predictor of initial hemoglobin level, followed by S. haematobium egg count and presence of malarial infection. Treatment with a single dose of MT or PR can produce substantial decreases in S. haematobium infection 8 months later.