A 2.75-month-old male infant was referred to our institution due to persistent hip subluxation despite 3 weeks of treatment in a Pavlik harness. Harness treatment was continued for 2 more weeks until it was noted that his left upper extremity function, which had been normal previously, had changed. Radiographs showed no bony abnormality. A compression or traction injury to his brachial plexus from the downward pull of the harness straps, possibly related to his relatively large size, was believed to be causative. Use of the harness was discontinued immediately, and his upper extremity motor function quickly returned to normal.