A poor diet and a lack of exposure to sunlight are common in patients with a psychiatric disorder. Both factors play a major role in the development of vitamin D deficiency. The prevalence of vitamin D deficiency might be higher in psychiatric patients due to the symptoms of the psychiatric disorder itself. By way of illustration, we focus on two patients who were diagnosed with a severe vitamin D deficiency during admission to a psychiatric clinic. In addition, we give advice regarding the diagnosis and treatment of vitamin D deficiency.