AN 11-YEAR-OLD GIRL has asymmetrical breast development (Figure 1). Findings from physical examination revealed aplasia of the left breast and absence of the left nipple-areola complex. In addition, the left anterior axillary fold and the left pectoralis major muscle were absent. The left posterior axillary fold was well developed and the latissimus dorsi muscles were present bilaterally. The left thoracic rib cage was flattened. The right breast bud and nipple-areola complex appeared to be developing normally. Findings from the remainder of the physical examination, including the upper extremities, were normal. There was no family history of similar abnormalities.