Vitamin D deficiency is common in critically ill patients and has been associated with adverse outcomes. Although many interesting observational studies have been published, only four small randomized controlled trials have been conducted in this vulnerable population. Endpoints included bone turnover, inflammatory markers and safety/efficacy issues. However, all of these trials were underpowered to detect clinically relevant endpoints due to their small sample size. Therefore, future studies focusing on morbidity and mortality endpoints are necessary in order to clarify whether vitamin D deficiency is only a surrogate marker for disease severity or whether treatment with sufficiently large doses of vitamin D may improve patient outcome in an intensive care setting.