A 14-YEAR-OLD girl had severe, sharp mid-back pain for 3 weeks and nontender, gradually enlarging left chest wall swelling for 2 weeks. It was unrelieved by change of position. One month before symptoms started, she had fallen, striking her left chest. No bruise remained and the tenderness subsided; however, 2 weeks later, swelling began in the area of injury. She had lost 10 pounds over the last 2 months while dieting. She had no fever, cough, chills, sweats, joint pain, or rash. Last year, she lost a filling from a right upper molar, but otherwise, her last dental care was 6 years ago. She was receiving 325 mg of ferrous sulfate per day for iron deficiency anemia diagnosed 1 month earlier. Findings from physical examination revealed an obese female (height, 139.7 cm; weight, 83.3 kg). Her temperature was 38.5°C; pulse, 120 beats per minute; respirations, 20 breaths per minute; and blood pressure, 122/88 mm Hg. Spinal examination showed scoliosis but no vertebral or costal tenderness. A 4-cm mass was palpated over the left rib cage at the anterior axillary line in the 10th interspace. This mass was ovoid, nontender, soft, and cool to the touch without erythema. A large cavity was present in the upper left molar.