The serum levels of 1,25-(OH)2 vitamin D3 were assayed in samples from 12 adolescent patients with SLE. Subnormal levels were observed in 7 of these 12 patients. Low levels of the metabolically active polar metabolite of vitamin D3 may contribute to the development of osteopenia observed in this disease. The cumulative effects of the osteoporotic and anti vitamin D effects of long term steroid therapy in children with SLE may require the cautious administration of supplemental vitamin D.