Vitamin D deficiency may be one reason for the onset and development of osteoporosis. The aim of the present study was to determine the occurrence rates of hypovitaminosis D in an unselected group of individuals presenting with common medical conditions and hospitalized for long periods. Concentrations of 1,25-(OH)2 D3 were measured in 89 patients (38 males and 51 females). Mean age was 70 years. Thirty-eight patients were tested in the spring and the remaining 51 patients in the autumn. Vitamin D3 levels were significantly reduced in patients tested in the autumn (p < 0.001). The reason for this surprising observation may have been the small number of sunny days and the long hospital stays during the study period, differences in the composition of both groups and the fact that 1,25-(OH)2 D3 reflects the actual vitamin D3 levels rather than those of its reserve in the human body. The results obtained suggesting hypovitaminosis D in an unselected group of elderly patients hospitalized with common diseases, even after the summer season, suggest the need for general supplementation of this vitamin throughout the year, regardless of the risk factors.