There are few data on the relative effects of calcium supplementation with or without extra vitamin D on BMD in patients selected for low vitamin D status. The aim of this study is to evaluate the relative importance of vitamin D and calcium treatment on BMD and bone-related chemistry in elderly women with vitamin D insufficiency. Three hundred two elderly women (age, 77.2 +/- 4.6 yr) with serum 25(OH)D concentrations <60 nM participated in a 1-yr randomized, double-blind, placebo-controlled trial. All subjects received 1000 mg calcium citrate per day with either 1000 IU ergocalciferol (vitamin D(2)) or identical placebo (control). The effects of time and time treatment interactions were evaluated by repeated-measures ANOVA. At baseline, calcium intake was 1100 mg/d, and 25(OH)D was 44.3 +/- 12.9 nM; this increased in the vitamin D group by 34% but not the control group after 1 year (59.8 +/- 13.8 versus 45.0 +/- 13.3 nM, p < 0.001). Total hip and total body BMD increased significantly, and procollagen type I intact N-terminal propeptide (PINP) decreased during the study with no difference between the treatment groups (hip BMD change: vitamin D, +0.5%; control, +0.2%; total body BMD change: vitamin D, +0.4%; control, +0.4%; PINP change: vitamin D, -3.9%; placebo, -2.8%). Although the fasting plasma and urine calcium increased in both groups equally, there was no detectable change in serum PTH. The increase in 25(OH)D achieved with vitamin D supplementation had no extra effect on active fractional intestinal calcium absorption, which fell equally in both groups (vitamin D, -17.4%; control, -14.8%). In patients with a baseline calcium intake of 1100 mg/d and vitamin D insufficiency, vitamin D(2) 1000 IU for 1 year has no extra beneficial effect on bone structure, bone formation markers, or intestinal calcium absorption over an additional 1000 mg of calcium. Vitamin D supplementation adds no extra short-term skeletal benefit to calcium citrate supplementation even in women with vitamin D insufficiency.