Plasma 25-hydroxyvitamin D (25(OH)D) concentrations in a group of thirtyeight nursing-home residents (age range 55-95 years) living in a sub-tropical climate (Brisbane, Australia) were less than 25 nmol/l in 50% of cases. The effects of endogenously and exogenously derived vitamin D have been studied over three months (April-June) using the following treatment groups: A, placebo; B, daily sun exposure; C, 18.8 nmol vitamin D/day and D, 56.3 nmol vitamin D/day. Exposure to ultra-violet radiation of group B patients was monitored by meter and palysulphone film. one month of treatment and groups B and C, respectively, after two months (P