Introduction: Vitamin D plays a critical role in immune function, and its deficiency has been linked to an increased risk of respiratory infections. This study investigates the association between serum 1,25-dihydroxyvitamin D levels and respiratory infections in hospitalized children. Methods: This case-control study was conducted at Heshmatieh Hospital, Sabzevar, Iran, involving 170 children (85 cases, 85 controls) aged 1 month to 16 years. The case group included children diagnosed with lower respiratory tract infections (LRTIs), while the control group comprised children hospitalized for non-infectious conditions. Serum 1,25-dihydroxyvitamin D levels were measured using ELISA, and inflammatory markers (WBC count, PMN percentage, ESR, and CRP) were analyzed. Chest X-ray (CXR) findings were evaluated. Results: Children with LRTIs had significantly lower serum vitamin D levels compared to controls (P = 0.021). Vitamin D deficiency (<30 nmol/L) was observed in 64.7% of cases and 32.8% of controls (P = 0.03). Inflammatory markers were elevated in cases, and CXR abnormalities were found in 74.2% of pneumonia patients, with bronchopneumonia being most common (61.8%). No significant difference in vitamin D deficiency between viral and bacterial pneumonia was detected (P = 0.54). Conclusions: This study highlights a significant association between lower serum vitamin D levels and respiratory infections in children, emphasizing the potential role of vitamin D in immune defense. Screening and supplementation strategies may be beneficial for at-risk pediatric populations.