A 7 month-old infant of Italian origin, who was given a vitamin-D deficient diet developed a Von Jaksch-Luzet syndrome with more than 20% myeloblasts in the bone marrow. Plasma levels of vitamin D metabolites wer low and parathormone levels were high. A low dose of vitamin D3 (2,000 IU cholecalciferol/day) resulted in the recovery of a normal phosphocalcic balance and in the disappearance of myeloblastosis.