The Jolivert Safe Water for Families program has sold sodium hypochlorite solution (chlorine) and conducted household visits in rural Haiti since 2002. To assess the impact of the program on diarrheal disease, in 2010 we conducted a survey and water quality testing in 201 program participants and 425 control households selected at random. Fifty-six percent of participants (versus 10% of controls) had free chlorine residuals between 0.2 and 2.0 mg/L, indicating correct water treatment. Using intention-to-treat analysis, we found that significantly fewer children < 5 in participant households had an episode of diarrhea in the previous 48 hours (32% versus 52%; P < 0.001) with 59% reduced odds (odds ratio = 0.41, 95% confidence interval = 0.21-0.79). Treatment-on-treated estimates of the odds of diarrhea indicated larger program effects for participants who met more stringent verifications of participation. Diarrheal disease reduction in this long-term program was comparable with that seen in short-term randomized, controlled interventions, suggesting that household chlorination can be an effective long-term water treatment strategy.