Copeptin has been associated with the severity of pneumonia and its complications. This study was designed to assess the usefulness of copeptin measurement in children with community-acquired pneumonia (CAP) and copeptin's relation with disease severity and sodium equilibrium. The study encompassed 311 patients (227 with pneumonia and 84 healthy controls) aged 8 days-18 years. Clinical findings and inflammatory markers were used to predict the disease severity. We found that the level of copeptin was significantly higher in patients with CAP (median 0.88 ng/mL) vs. healthy children (0.33 ng/mL; p < 0.01). ROC analysis showed a high AUC value (0.87) and the cut-off point for plasma copeptin level was 0.44 ng/mL, with a high sensitivity (89 %) and specificity (73 %) in recognizing pneumonia. Patients with higher copeptin concentrations were at higher risk of hyponatremia (OR 2.43). Yet there was only a weak reverse correlation between the sodium and the copeptin concentrations (Spearmann's rank coefficient = -0.19). The levels of copeptin were higher in hyponatremic patients (0.83 ng/mL) vs. normonatremic patients (0.69 ng/mL; p = 0.02). Copeptin elevation did not reflect the CAP severity measured with traditionally used methods. In conclusion, copeptin elevation is a promising marker of pneumonia, but it reflects neither the disease severity nor sodium concentration.