Background: Acute diarrhea results in an estimated 2.1 million child deaths annually and considerable global morbidity. The WHO Global Burden of Disease initiative has adopted a health status measurement known as ‘‘disability-adjusted life-year’’, a unifying metric that combines mortality and morbidity. In 2002, diarrheal diseases ranked second among conditions afflicting children. Despite the disease burden, there appears to be limited consensus on how to best measure pediatric acute diarrhea. Objectives: To determine the validity and reliability of primary outcome measures used in randomized controlled trials (RCTs) involving pediatric acute diarrhea. Methods: A systematic review was performed of four major databases published in English from inception to February 2007. RCTs in children < 19 years of age measuring acute diarrhea as a primary outcome were included. Descriptive statistics were employed to illustrate the characteristics of trials measuring acute diarrhea as their primary outcome. Results: We identified 117 RCTs reporting one or more primary outcomes related to acute diarrhea. Trials reported 59 different primary outcomes. The most frequent primary outcome was duration of diarrhea (n = 64 trials). However, among all included trials, there were 42 different definitions of acute diarrhea (i.e. inclusion criteria for treatment trials, diagnostic criteria for prophylaxis trials) and 59 definitions of what constituted diarrhea resolution. Although no reports defined diarrhea differently among different pediatric age groups, 14 trials stratified by age group. Thirty-two trials used scoring systems (e.g. scales), yet only one cited prior clinometric properties. Conclusions: According to published trials, it appears that none of the primary endpoints in current use have been properly validated. This observed deficiency may be a result of authors neglecting to report the measurement properties of the primary outcomes in the trials we reviewed. If so, future trials need to improve the quality of their reporting. In addition, there is limited consensus on the definition of pediatric acute diarrhea, its remission, or criteria for evaluating its severity. Clinometric properties (i.e. validity, reliability) are critical if clinicians and public health personnel wish to make valid inferences regarding the efficacy of different interventions for pediatric acute diarrhea.