Study objectives: We define the predictive value of parents' computer-based report for history and physical signs of dehydration for a primary outcome of percentage of dehydration (fluid deficit) and 2 secondary outcomes: clinically important acidosis and hospital admission. We also sought to compare the reports of physical signs related to dehydration made by parents and nurses. Methods: We performed a prospective observational trial in an urban pediatric emergency department. A convenience sample of parents completed a computer-based interview covering historical details and physical signs (ill appearance, sunken fontanelle, sunken eyes, decreased tears, dry mouth, cool extremities, and weak cry) related to dehydration. Nurses independently completed an assessment of physical signs for enrolled children. The primary outcome was the degree of dehydration (fluid deficit), which was defined as the percentage difference between initial ED weight and stable final weight after the illness. Secondary outcomes included clinically important acidosis (defined as a serum CO₂ value of ≤15 mEq/L) and hospital admission. Results: One hundred thirty-two parent-child dyads comprised the final sample. Parent-reported data manifested higher sensitivity (range 73% to 100%) than specificity (range 0% to 49%) for the prediction of dehydration of 5% or greater. Likelihood ratios (LRs) near zero (<0.1) suggest that a normal history of fluid intake and urine output reduced the likelihood of significant dehydration. Parental report of a normal tearing state reduced the likelihood of significant dehydration and clinically important acidosis (negative LRs of 0.4 and 0.1, respectively). Two physical signs reported by parents, sunken fontanelle and decreased tears, were associated with hospital admission (positive LR of 3.4 and 4.0, respectively). Conclusion: Parents' report of history and observations for children captured through computer-based interview demonstrates predictive value for relevant outcomes in dehydration. [Ann Emerg Med. 2003;41:196-205.]