Serum 25-hydroxyvitamin D (25-OHD) levels were measured in 338 elderly patients admitted to the Geriatric Medicine Departments of a general hospital in Israel in the course of one year. The mean (+/- SD) serum 25-OHD levels were significantly lower (P less than .01) in the elderly patients (13.5 +/- 8.9 ng/mL) than in healthy young controls (24.7 +/- 6.1 ng/mL). One hundred ten patients (35.5%) were either vitamin D deficient (25-OHD less than 5 ng/mL) or had borderline serum levels of 25-OHD (5-9 ng/mL). The mean (+/- SD) serum 25-OHD concentration of patients who were completely mobile before hospitalization was 15.5 +/- 8.8 ng/mL (n = 239). In patients mainly immobilized but able to leave the house occasionally, it was 10.2 +/- 6.3 ng/mL (n = 84) and of bed-ridden patients, it was 5.2 +/- 3.2 ng/mL (n = 15). No correlation was found between serum 25-OHD levels and the patients' age or serum calcium, phosphorus, alkaline phosphatase, and albumin values. Thus, in order to detect vitamin D deficiency in the elderly, it is necessary to measure serum 25-OHD concentration. The results demonstrate that vitamin D deficiency is common among elderly patients even in sunny climates and indicate the need for development of effective programs of prevention and treatment.