Objectives: A significant proportion of the older population may exhibit vitamin D insufficiency. We sought to establish the proportion of 25-hydroxyvitamin D (25OHD) insufficient individuals in an older female cohort presenting for acute medical admission and how they responded to supplementation. Design: A prospective cohort study. Setting: Hospital admissions followed up as a population-based study. Subjects: A total of 114 consecutive female acute medical admissions aged over 65 years from November 2003 to January 2004 were enrolled. All admissions with hypercalcaemia, metabolic bone disease (other than osteoporosis/osteomalacia) and creatinine 4150 mmol/l were excluded. Interventions: iPTH, calcium and 25OHD levels were measured in each patient. Of the total, 22 were already receiving calcium and vitamin D supplementation at enrolment. The remaining 92 were commenced on 800 IU of vitamin D and 1 g calcium, and levels were reassessed after supplementation for 3 months. Results: 25-Hydroxyvitamin D insufficiency, as defined by a 25OHD concentration of o50 nmol/l, was present in 86 (75.4%) patients at initial assessment (mean 35.8 nmol/l, s.d. 23.3). Secondary hyperparathyroidism was present in only 36.7% of those with 25OHD deficiency at baseline. Of the total, 51 (44.7%) patients presented for follow-up. 25-Hydroxyvitamin D concentration increased in this group from 42.1 nmol/l (s.d. 26.6) to 59.5 nmol/l (s.d. 27.4) after supplementation, Po0.001, but 18(35.3%) still remained deficient. There was no significant change in iPTH or calcium following supplementation. Assessment of compliance revealed 6 (11.7%) admitted to partial or non-compliance. Conclusions: Insufficiency of 25OHD was very common in this cohort. Despite calcium and vitamin D supplementation, 25OHD concentrations failed to reach normality in a significant proportion. Maintaining vitamin D and calcium intake at the level of current recommended doses may not be sufficient to ensure adequate 25OHD stores.