To date, no study has investigated the nutritional status of vitamin D in frail elderly people living at home. The purposes of this study were to assess serum 25-hydroxyvitamin D (25[OH]D) levels and associated factors in noninstitutionalized elderly people who had various levels of physical disability, and to propose an adequate vitamin D nutritional status for the elderly by interpreting the serum 25(OH)D levels in relation to serum parathyroid hormone (PTH) levels in this population. Health examinations were conducted in the winter and summer of 2003. The subjects were 143 elderly people in the winter, and 120 elderly people in the summer, who all used the long-term care insurance system at home. Serum 25(OH)D concentrations were determined with a chemiluminescence protein-binding assay, and serum intact PTH concentrations were determined with an immunoradiometric assay. The subjects' disease histories and lifestyle information were obtained through an interview. Activities of daily living (ADL) levels were evaluated using the Barthel index, and grip strength was measured with a digital hand dynamometer. Average serum 25(OH)D levels in the winter and summer were 54.2 nmol/l (SD 29.0) and 53.3 nmol/l (SD 32.3), respectively, and intact PTH concentrations in the winter and summer were 4.2 pmol/l (SD 1.8) and 4.3 pmol/l (SD 1.8), respectively. The proportion of people who had a low 25(OH)D (<30 nmol/l) and high intact PTH levels (>6.9 pmol/l) were 15%-20% and 8%, respectively. Significant predictors of low serum 25(OH)D concentrations were low ADL levels, female sex, and low fish consumption in both seasons. Serum 25(OH)D concentrations of less than 50 nmol/l were associated with elevated serum intact PTH concentrations. In conclusion, elderly people requiring care at home are at high risk of hypovitaminosis D, and their low serum 25(OH)D levels are mainly associated with low ADL levels. In addition, maintenance of serum 25(OH)D concentrations above 50 nmol/l may prevent hypovitaminosis D-induced hyperparathyroidism.