In the VITdAL-ICU trial using a large oral dose of vitamin D3 in 480 adult critically ill patients, there was no benefit regarding the primary endpoint hospital length of stay. However, the predefined subgroup with severe vitamin D deficiency (25(OH)D ≤ 12ng/ml) had significantly lower 28-day mortality (36.3% placebo vs. 20.4% vitamin D group, HR 0.52 (0.30-0.89), number needed to treat = 6). Therefore, high-dose vitamin D3 in a population of severely vitamin D deficient critically ill patients is a promising and inexpensive intervention that requires confirmatory multicenter studies.