Background: The prevalence of vitamin D deficiency in critically ill patients has been reported to be as high as 80%. There is insufficient data regarding the relationship between 25-hydroxyvitamin D [25(OH) D] levels and outcomes in medical intensive care unit (MICU). The goal of this study was to evaluate the prevalence of 25(OH) D deficiency in MICU and its relationship with outcomes. Subjects and Methods: This was a retrospective study in a MICU of a teaching medical college hospital of Eastern India. All patients admitted to MICU, who had levels of 25(OH) D available, were included in the study. The discriminative powers of admission and lowest 25(OH) D values regarding day-30 mortality were evaluated by producing receiver operating curves (ROC). Binary end points were analyzed by means of a Fisher′s exact test. Continuous variables were compared by using unpaired t-tests, Welch′s tests, or Wilcoxon ranksum tests. All odds ratios and their corresponding 95% confidence intervals were calculated according to the profile-likelihood method. The time from inclusion to death in the two groups was compared with the use of the log-rank test, and the results are presented as Kaplan-Meier curves. Hazard ratios for death from hypo 25(OH) D were calculated by logistic regression model. All P values were 2-tailed and P < 0.05 was considered statistically significant. Results: Of the 300 patients admitted during the study period, 25(OH) D levels were available in 152 patients (50.6%). Of these 152 patients, 15 patients (9.8%) had 25(OH) D insufficiency (20-29.9 ng/dL), 79 (51.9%) had 25(OH) D deficiency (0-19.9 ng/dL), and the levels were normal (>30 ng/dl) in 58 (38.2%) patients. Most of the patients with deficient 25(OH) D levels were females (P < 0.05). Higher mortality (P = 0.01), increased length of MICU stay, and prolonged ventilation were observed in patients with 25(OH) D deficiency. Conclusions: Patients with 25(OH) D deficiency in MICU have increased hospital mortality, longer mechanical ventilation, and longer MICU stay.