BACKGROUND Vitamin D deficiency is common in various populations worldwide. Adverse effects of vitamin D deficiency are the development of bone disorders; however, other diseases such as multiple sclerosis, type 1 diabetes, rheumatoid arthritis and certain cancers have also been linked to vitamin D deficiency. The general medical inpatient population is a group at increased risk of vitamin D deficiency. These patients often have coexistent risk factors for its consequences. This study aims to document a point prevalence of vitamin D deficiency in this population. METHODS Two cross-sectional audits of patients admitted to general medicine units were carried out--the first in mid-November at the end of winter and the second in mid-April and May at the end of summer. Information regarding patients' comorbidities, medication usage, previous falls and fractures was obtained and serum 25-hydroxyvitamin D, parathyroid hormone and calcium levels were measured. RESULTS A total of 129 patients was studied (65 in winter and 64 in summer). Ninety-four patients (74%) had 25-hydroxyvitamin D levels < or = 50 nmol/L. Seven patients had severe deficiency (levels < or = 12.5 nmol/L). Average vitamin D levels were lower at the end of winter (35 vs 43 nmol/L, P = 0.007). Of the 37 patients receiving vitamin D supplements, 20 (54%) had 25-hydroxyvitamin D levels < or = 50 nmol/L. CONCLUSIONS Low vitamin D levels were common in this general medical inpatient population. The average vitamin D level was lower in the patient group tested in November following winter. Supplementation of vitamin D did not uniformly prevent deficiency.