OBJECTIVES: Recent large trials indicate that adherence associated with a daily regimen of vitamin D is low and limits anti-fracture efficacy with vitamin D supplementation. The aim of this report is to describe changes of 25-hydroxyvitamin D (25(OH)D) serum concentrations achieved with a single oral dose of 300000 IU vitamin D3. METHODS: Over a course of 4 months, we identified 33 elderly with severe vitamin D deficiency (25(OH)D<25 nmol/l) on admission to acute care. Patients were admitted for musculoskeletal pain, bone disease, or gait abnormalities. The mean age was 80.5 years (SD+/-6.1). All patients were treated with a single oral dose of 300000 IU D3 in combination with 500-1000 mg calcium supplements per day depending on their dietary calcium intake. RESULTS: Baseline mean 25(OH)D serum concentrations were 15 nmol/l (SD+/-5.5). Mean 25(OH)D serum concentrations increased to 81.4 nmol/l (SD+/-29.7) at 3 months (29 patients) and were still 69.0 nmol/l (SD+/-17.9) at 6 months (26 patients). Mean serum calcium levels were 2.24 mmol/l (SD+/-0.11) at baseline, 2.28 mmol/l (SD+/-0.18) at 3 months, and 2.28 mmol/l (SD+/-0.13) at 6 months. Two patients with mild hypercalcemia (2.69 mmol/l) at 3 months had normal values at 6 months. CONCLUSION: Based on our observations, a single oral dose of 300000 IU vitamin D3 raises mean 25(OH)D serum concentrations to the target mean of above 75 nmol/l at 3 months and a mean level of 69 nmol/l at 6 months. As calcium absorption is enhanced with higher 25(OH)D serum concentrations, calcium supplementation may need downward adjustment with this regimen to avoid mild hypercalcemia.