Objective: The purpose of this study was to assess the prevalence of an elevated anion gap in children between the ages of 6 and 61 months treated in the Emergency Department for dehydration.Methods: This study was a retrospective chart review. The hospital database was queried to find patients between the ages of 6 and 60 months of age with a diagnosis of dehydration. The patient records were then reviewed to determine which patients had electrolytes, BUN, and creatinine obtained as part of their Emergency Department evaluation. All patients who had these tests performed were included in the study. The patients were grouped into the following categories based on serum CO2; 0-11, 12-15, 16-19, and over 20meq/L. The percentage of patients with an elevated anion gap (elevated>16) was then calculated for each of these groups. Spearman correlation coefficients were also calculated between the following factors; age, CO2, anion gap, BUN, creatinine, and glucose. Results: During the period of this study, 197 patients with the diagnosis of dehydration and the specified laboratory data were enrolled in this study. The mean age of these patients was 26.6 months (SD=14.6) with a range of 6 to 61 months. The mean serum CO2 was 18.5(SD=4.94). When the patients were divided into groups based on the CO2, we found elevated anion gaps in 12.5% of patients with a CO2>20, 56% in patients with CO2 of 16-19, 91% when the CO2 was 12-15, and 97% when the CO2 was 11 or less. The anion gap had a high negative correlation with the serum CO2 (r=-0.70, p=0.0001) and a moderate negative correlation with serum glucose(r=-0.51, p=0.0001). Conclusions: An anion gap is common in patients between the age of 6 and 61 months presenting to the ED with dehydration and there is a significant correlation between the CO2 and the anion gap.