One-day-old Leghorn chicks fed a copper-deficient diet of whole dried milk and sucrose became lame after 2 to 4 weeks. The bones were fragile and easily broken, but were not grossly deformed. Microscopically, the epiphyseal cartilage was thickened to a variable degree through an increase in immature non-calcified cartilage. Vascular invasion of the thickened cartilage was irregular and less extensive than in control chicks. In the metaphysis only a few scattered and fragmented spicules were observed Osteoid tissue was minimal to absent. The cortical bone of the shaft was thinner and less dense, and there was evidence of increased resorption. The lesions were substantially different from those described in mammals deprived of Cu, and closely resembled the osseous alterations reported for vitamin A deficiency. Serum inorganic P, Ca, and alkaline phosphatase did not differ significantly among chicks on the various diets. Ascorbic acid did not reduce the incidence or severity of the skeletal lesions.