Growth arrest complicating unicameral bone cyst of the proximal humerus is reported in five patients. These patients represented 10% of 51 consecutive patients treated by the senior author at our hospital from 1988 through 1995. The youngest patient at first clinical presentation was aged 6 years, and the oldest was 14 years 9 months. The follow-up ranged from 35 to 69 months(average, 57.2). The youngest patient at final follow-up was 9 years 1 month, and the oldest was 20 years 5 months. Pathologic fracture was the common clinical presentation in all patients. Growth arrest was diagnosed by limb-length discrepancies, as well as radiographic evidence of premature closure of the physis and deformity of the upper humerus. Treatment was either aspiration of the cyst and local injection of corticosteroids or curettage and bone grafting. Growth arrest was documented, both clinically and radiographically, before the surgery in the two cases treated by curettage and grafting. The origin of growth arrest resulting from unicameral bone cyst remains uncertain. Direct iatrogenic damage to the physis was not a likely cause of growth arrest in this series. Growth arrest as a complication of unicameral bone cyst of the proximal humerus is more common than is generally appreciated (10%).