Vitamin D plays an important role in maintaining normal bone metabolism. Recent studies have suggested that vitamin D influences many other physiological processes, including muscle function, cardiovascular homeostasis, nerve function, and immune response. Furthermore, accumulated evidence suggests that vitamin D also mediates the immune system response to infection. Critical neurosurgical patients have higher infection and mortality rates. To correlate vitamin D deficiency to the immunological status of neurosurgical intensive care unit patients, we detected serum vitamin D level in 15 patients with clinically suspected infection and 10 patients with confirmed infection. Serum level of 25-hydroxyvitamin D, the primary circulating form of vitamin D, was significantly decreased in patients with confirmed infection after a 2-week neurosurgical intensive care unit hospitalization, while serum level of 1,25-dihydroxyvitamin D, the active form of vitamin D, was significantly decreased in patients after a 4-week neurosurgical intensive care unit hospitalization. Levels of both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D were in the deficient state after 7 and 14 days in the neurosurgical intensive care unit, respectively, in patients with clinically suspected infection. These findings suggest that vitamin D deficiency is linked to the immunological status of neurosurgical intensive care unit patients and vitamin D supplementation can improve patient’s immunological status.