A series of 175 unicameral bone cysts is presented. The pathogenesis of the lesion and the results of treatment differed depending upon the specific bone involved. It is our opinion that prompt surgical treatment to establish an accurate histological diagnosis and to reinforce the involved segment of bone against fracture by thoroughly evacuating the cavity and filling it with bone grafts is a more satisfactory treatment than watchful waiting. A persistent and static roentgenographic defect after surgery should not be considered a clinical failure if adequate bone strength is present. A second operation is indicated only for an enlarging cyst with the threat of fracture. In this series of 129 primary operations, the incidence of re-operation was 30 per cent in the proximal end of the humerus, 17 per cent in the proximal end of the femur, 1 1 per cent in the proximal end of the tibia, and nil in most of the other less frequent locations. In retrospect, a number of second procedures were considered unnecessary since they were performed for asymptomatic and static roentgenographic defects associated with sufficient bone strength to prevent fracture. True recurrence followimug surgical treatment is significantly more frequent in patients under ten years of age. Age is a more reliable prognostic criterion than the proximity of the cyst to the epiphyseal plate when assessing the likelihood of recurrence after surgical treatment.