A hospital based case-control study for assessing the effectiveness of oral rehydration therapy (ORT) preparation against severe dehydration due to diarrhea was conducted at the Infectious Diseases Hospital, Jakarta, Indonesia. A total of 202 children aged 24 months or less who attending the hospital were suffering from acute watery diarrhea were recruited in the study. Those who were severely dehydrated as assessed by WHO criteria were accounted as cases; those who were non-severely dehydrated were accounted as controls. There were 59 cases and 143 controls. A questionnaire was used to interview all study subjects' mothers about ORT usage and various risk factors. Mothers who used ORT were asked to show how they prepared either oral rehydration solution (ORS) or sugar salt solution (SSS). Effectiveness of ORT against severe diarrheal dehydration was based on the formula for assessment of vaccine efficacy by using the odds ratio (OR). With the use of the logistic regression method, an adjusted OR was obtained after controlling various confounders. The effectiveness of ORT against severe diarrheal dehydration was 72.1% for proper ORT preparation and was decreased to 63.2% when ORT was improperly prepared.