A study was done to estimate the prevalence of hypoglycemia among children with dehydration due to acute gastroenteritis, and to identify clinical variables associated with hypoglycemia in these children. A retrospective case series of children older than 1 month of age and younger than 5 years of age who presented to an urban children's hospital Emergency Department with acute gastroenteritis and dehydration was performed. Medical records were reviewed; demographic and clinical data, including pretreatment serum glucose concentrations, were recorded. There were 196 children comprising the study population. Eighteen children (9.2%) were hypoglycemic. The duration of vomiting was longer for the children with hypoglycemia (2.6 days, SD +/- 1.5) than for those without hypoglycemia (1.6, SD +/- 1.8), 95% CI 0.13 to 1.88. Hypoglycemia may complicate dehydration due to acute gastroenteritis in young children. Clinicians should examine the serum glucose concentration in these children.