The etiology of congenital pseudarthrosis of the clavicle remains obscure. The authors treated a 5-year-old boy with this rare disease. Clinical history and radiographs confirmed the diagnosis. Surgical treatment of resection of the pseudarthrosis, autologous bone graft, and internal fixation with a small AO plate led to successful bone union. Histologic analysis of the pseudarthrosis showed that there were cartilaginous caps on the proximal and distal bony ends. The appearance of the cartilaginous caps were equivalent to that of developing physes. They were adding new bone to the bony ends, which also was confirmed by preoperative tetracycline labeling. These observations support the hypothesis that the pseudarthrosis is caused by failure of 2 ossification centers to fuse, as was previously proposed by others.