Purpose: A growing number of evidence demonstrates deficiency of vitamin D in critically ill patients. We aimed to evaluate the vitamin D status of our critically ill patients and its relevance to infections in these patients. Material and Methods: We conducted a prospective observational study in 201 critically ill patients admitted to the medical intensive care unit of Gazi University Hospital between October 2009 through March 2011. Results: Sixty-nine percent of the patients were found to be vitamin D deficient. Infection rate was higher in the deficient group, though without statistical significance (P = .117). Infections with Acinetobacter baumannii was significantly more frequent in patients with Vitamin D deficiency (25% vs 10%, P = .012). The median level of 25-hydroxyvitamin D levels was 11.8 [6.3-17.2] ng/mL and 15.7 [8.1-28.9] ng/mL in patients with and without A baumannii infections respectively (P = .024). Logistic regression analysis demonstrated that vitamin D deficiency (P = .042) and invasive mechanical ventilation (P = .001) were the 2 independent risk factors in the development of A baumannii infections, in addition. Conclusions: Vitamin D deficiency is common in critically ill patients. Even though there was no statistical difference between vitamin D deficient and sufficient patients regarding development of infections in general, A baumannii infections were significantly more frequent in the deficient group. Vitamin D deficiency was found as one of the independent risk factors for A baumannii infections. Further multicenter studies with a larger sample size are required to validate our data. © 2013 Elsevier Inc.