Objective: To define the osmol gap (OG) range in pediatric ED (PED) patients. Methods: This was a blinded, observational patient series involving an urban PED with an annual census of 35,000. All patients presenting to the Hasbro Children's Hospital Emergency Department who required electrolyte determination during their evaluations were enrolled into the study. Exclusionary criteria included the presence of urinary ketones, alcohol ingestion within the preceding 24 hours, or illnesses that are known to change serum osmolarity. Electrolytes, BUN, glucose, and freezing point depression osmolality were measured on a single serum specimen. Additional laboratory information included ethanol and anion gap. The OG was determined using each of three equations previously described in the literature (see Results). The best coefficients for sodium, BUN, and glucose were determined by multiple linear regression. Results: 192 children (90 girls, 102 boys) with a median age of 6.6 years (mean: 7.3 years; range: 7 days to 17.9 years) made up the study population. The mean measured osmolality (± SD) for the entire sample was 284.2 ± 6.9 mOsm/dL with a range of 265-311 mOsm/dL. Mean osmol gaps with standard deviations varied with the equation used for calculation. Conclusion: Regardless of the equation used, the range of “normal” osmol gaps in the pediatric population is approximately 22 mOsm.