Objective: Serum sodium levels change in patients with lung infections. This study aimed to evaluate serum sodium levels in patients who were admitted to the hospital with a lower respiratory tract infection (LRTI) and to assess its association with C-reactive protein (CRP) levels.Material and Methods: A cross-sectional study was designed. A total of 549 inpatients (207 females, 37.7%; 342 males, 62.3%) aged 1-15 years who were diagnosed with LRTI (pneumonia, bronchitis, and bronchiolitis) at the Kars State Hospital's pediatric ward between 01.01.2011 and 30.06.2014 were included in the study. The records of these patients were retrospectively reviewed. Age, gender, white blood cell (WBC) count, absolute neutrophil count (ANC), sodium levels, and CRP levels were recorded. The patients were classified into five groups according to serum sodium levels as severe hyponatremia, moderate hyponatremia, mild hyponatremia, normonatremia, and hypernatremia. Results: There was no significant difference for age or gender between the hyponatremia and normonatremia groups. There was one patient with severe hyponatremia (<=125 mmol/L), nine with moderate hyponatremia (126-130 mmol/L), 90 with mild hyponatremia (131-134 mmol/L), 446 with normonatremia (135-145 mmol/L), and three with hypernatremia (>=146 mmol/L). There was a negative correlation between serum sodium levels and CRP levels (r=-0.178; p<0.001), WBC counts (r=-0.121; p=0.004), and ANC (r=-0.100; p=0.048). Conclusion: There was a statistically significant negative correlation between serum sodium levels and acute-phase reactants in patients with pneumonia. Serum sodium levels must be closely monitored in these patients, particularly when acute-phase reactant levels are high.