['Our understanding of the influence of a low plasma-25(OH) vitamin D3 (25-OHD) level on psychiatric disease is growing. Very limited information is available about the 25-OHD level in psychiatric populations. This study was initiated to determine which patients should have their 25-OHD levels analysed and who would require treatment.', 'This retrospective, cross-sectional study comprised patients admitted for hospitalisation at Mental Health Centre Frederiksberg from 25 May to 9 September 2010. A total of 170 patients and their corresponding 25-OHD results were included.', 'Of the 170 patients, 55% (n = 93) were women and 45% (n = 77) were men. Thirteen patients (8%) had severe to moderate 25-OHD deficiency, 59 had insufficiency and 98 had a normal 25-OHD. In total, 42% of the results were abnormal. No differences were detected according to sex, age or diagnosis group. No correlation was found between 25-OHD and cobalamine, thyroid-stimulating hormone or Ca2+.'] Are vitamin D plasma levels during summer in a psychiatric population comparable to the winter levels of healthy individuals?