BACKGROUND: Although the pentavalent rotavirus vaccine was highly efficacious against rotavirus diarrhea in clinical trials, the effectiveness of vaccine under field conditions in the developing world is unclear. In October 2006, Nicaragua became the first developing nation to implement universal infant immunization with the pentavalent rotavirus vaccine. To assess the effect of the immunization program, we examined the incidence of diarrhea episodes between 2003 and 2009 among children in the state of León, Nicaragua. METHODS: We extracted data on diarrhea episodes from health ministry records. We used scaled Poisson regression models to estimate diarrhea incidence rate ratios for the period following the program's implementation to the period before implementation. RESULTS: Following implementation of the immunization program, diarrhea episodes among infants were reduced (incidence rate ratios: 0.85, 95% confidence interval: 0.71-1.02) during the rotavirus season, but appear to have increased during other months. CONCLUSIONS: Although the immunization program appears effective in reducing diarrhea episodes during the rotavirus season, a large burden of diarrhea still persists during the remainder of the year. © 2011 Lippincott Williams & Wilkins, Inc.