© 2020 French Society of Pediatrics Introduction: Potassium abnormalities are frequent in intensive care but their incidence in the emergency department is unknown. Aim: We describe the spectrum of potassium abnormalities in our tertiary-level pediatric emergency department. Methods: Retrospective case-control study of all the patients admitted to a single-center tertiary emergency department over a 2.5-year period. We compared patients with hypokalemia (< 3.0 mEq/L) and patients with hyperkalemia (> 6.0 mEq/L) against a normal randomized population recruited on a 3:1 ratio with potassium levels between 3.5 and 5 mEq/L. Results: Between January 1, 2013 and August 31, 2016 we admitted 108,209 patients to our emergency department. A total of 9342 blood samples were tested and the following potassium measurements were found: 60 cases of hypokalemia (2.8 ± 0.2 mEq/L) and 55 cases of hyperkalemia (6.4 ± 0.6 mEq/L). In total, 200 patients with normokalemia were recruited (4.1 ± 0.3 mEq/L). The main causes of the disorders were non-specific: lower respiratory tract infection (23%) and fracture (15%) for hypokalemia, lower respiratory tract (21.8%) and ear–nose–throat infections (20.0%) for hyperkalemia. Patients with hyperkalemia had an elevated creatinine level (0.72 ± 1.6 vs. 0.40 ± 0.16 mg/dL, P < 0.0001) with lower bicarbonate (19.4 ± 3.8 vs. 21.8 ± 2.8 mmol/L, P = 0.0001) and higher phosphorus levels (1.95 ± 0.6 vs. 1.42 ± 0.27 mg/dL, P = 0.0001). Patients with hypokalemia had an elevated creatinine level (0.66 ± 0.71 vs. 0.40 ± 0.16 mg/dL, P < 0.0001) and a lower phosphorus level (1.12 ± 0.31 vs. 1.42 ± 0.27 mg/dL, P = 0.0001). We did not observe significant differences in pH, PCO2, base excess and lactate, or in the mean duration of hospitalization in general wards and pediatric intensive care units according to the PIM and PRISM scores. Discussion: Dyskalemia is rare in emergency department patients: 0.64% for hypokalemia and 0.58% for hyperkalemia. This condition could be explained by a degree of renal failure due to transient volume disturbance. The main mechanism is dehydration due to digestive losses, polypnea in young patients, and poor intake. In the case of hypokalemia, poor intake and digestive losses could be the main explanation. These disorders resolve easily with feeding or perfusion and do not impair development. Conclusion: Dyskalemia is rare in emergency department patients and is easily resolved with feeding or perfusion. A plausible etiological mechanism is a transient volume disturbance. Dyskalemia is not predictive of poor development in the emergency pediatric population.