Low vitamin D status common in the elderly could result in hypocalcemia and secondary hyperparathyroidism. In the present study, however, in geriatric patients with very low serum 25-hydroxy-vitamin D concentration, no correlation was found between serum ionized calcium and vitamin D status. Furthermore, supplementation over 40 weeks with vitamin D (1000 U/day) and/or calcium (1 g/day) did not affect ionized calcium levels. However, intact parathyrin levels (PTH) were decreased (32 to 27 ng/l, p < 0.05) by vitamin D, whereas they were increased 38 to 42 ng/l, p < 0.05) in the placebo group. The levels of calcitriol were increased by vitamin D (89 to 105 pm/l, p < 0.05). Calcium had no significant effects. One can conclude that in chronically bedridden geriatric patients with low vitamin D status, (i) vitamin D supplementation lowers serum PTH, and (ii) the serum ionized calcium concentration is not a sensitive indicator of the low vitamin D status.