Background: In most developed countries overt vitamin D deficiency, characterized by rickets or osteomalacia, is now uncommon. However, subclinical vitamin D insufficiency is extremely common and may contribute to the development of skeletal and non-skeletal problems. Standard practice involves supplementation with a combination of vitamin D and calcium although the benefit of adding calcium to vitamin D supplements has not been fully established and may reduce adherence due to its bulky and chalky consistency. Purpose of study: To compare the effects of vitamin D alone versus vitamin D/calcium supplements on vitamin D levels, bone profile and parathyroid hormone level. Population: Older (>65 years) female patients living in the community and long term care institutions. Interventions: Either 800 iu of vitamin D3 or a composite supplement of 800 iu vitamin D3 and 1000mg calcium were given to patients in an open-labelled observational study. Serum 25-hydroxy-vitamin D, parathyroid hormone, calcium, phosphate and alkaline phosphatase levels were assessed at baseline and after 3 months of treatment. Results: Serum 25-hydroxy-vitamin D levels rose from baseline levels of 25 +/- 16 to 79 +/- 16 in those treated with vitamin D alone and from 35 +/- 24nmol/L to 70 +/- 24 nmol/L in those treated with vitamin D and calcium. Serum PTH levels fell by similar amounts in both groups. In both community dwellers and institutionalised patients, those treated with vitamin D alone were at least as likely to achieve normalisation of serum vitamin D levels as those on combined calcium/vitamin D treatment. Conclusion: Vitamin D alone appears as effective as combined calcium/vitamin D treatment in restoring serum vitamin D levels in older community dwelling and institutionalised patients. A prospective randomised trial would help confirm these findings.