The variable clinical course and locally aggressive growth of giant cell tumours of bone cause problems in planning treatment. Curettage and bone grafting is the commonest form of surgical treatment, but recurrence rates as high as 40 per cent have been reported. Thirty-four patients with a GCT were treated in our clinic between years 1982 and 1990. There were three gr 1, 24 gr II and 7 grade III tumours. Operative treatment was carried out in 31. DNA flow cytometry was performed from the sample in 28 patients. The type of treatment played an important role in tumour prognosis: there were 10 recurrences after 18 intralesional procedures, but only one recurrence after tumour resection. DNA flow cytometry was not helpful in predicting tumour behaviour.