The mechanisms of action of vitamin D are the subject of intense investigation. Evidence now suggests vitamin D affects immune function and cell proliferation, prompting interest in its role in critical illness and cardiac disease. Multiple studies demonstrate strong associations between vitamin D deficiency and severity of illness including need for higher inotrope support, more fluid resuscitation, and longer intensive care unit stay. The pediatric cardiac population may be at even more risk and nearly twice as likely to be deficient compared to the noncardiac population. Low vitamin D levels have been found in postoperative cardiac patients, where investigators speculate cardiopulmonary bypass alters levels directly or indirectly. Patients with congestive heart failure who are deficient also seem to benefit from vitamin D supplementation. This review summarizes recent studies in children that investigate the relation between vitamin D status and clinical outcomes in the critically ill including those with cardiac disease.