OBJECTIVES: We examined the impact of school water, sanitation, and hygiene (WASH) interventions on diarrhea-related outcomes among younger siblings of school-going children. METHODS: We conducted a cluster-randomized trial among 185 schools in Kenya from 2007 to 2009. We assigned schools to 1 of 2 study groups according to water availability. Multilevel logistic regression models, adjusted for baseline measures, assessed differences between intervention and control arms in 1-week period prevalence of diarrhea and 2-week period prevalence of clinic visits among children younger than 5 years with at least 1 sibling attending a program school. RESULTS: Among water-scarce schools, comprehensive WASH improvements were associated with decreased odds of diarrhea (odds ratio [OR] = 0.44; 95% confidence interval [CI] = 0.27, 0.73) and visiting a clinic (OR = 0.36; 95% CI = 0.19, 0.68), relative to control schools. In our separate study group of schools with greater water availability, school hygiene promotion and water treatment interventions and school sanitation improvements were not associated with differences in diarrhea prevalence between intervention and control schools. CONCLUSIONS: In water-scarce areas, school WASH interventions that include robust water supply improvements can reduce diarrheal diseases among young children.