A group of vitamin-D-depleted Asians was identified in the spring of 1980 and treated with 2.5 mg ergocalciferol in the autumn of that year. Treatment produced a sustained rise in the serum level of 25-hydroxyvitamin D which lasted until the spring of 1981. The response was more predictable after oral than after intramuscular administration. Treatment in the autumn provides an effective, efficient, and cheap means of prophylaxis against vitamin D deficiency in Asians during the winter. A "once a year" regimen promises a much higher compliance rate than could be achieved by daily supplementation.