Flow cytometric DNA analysis was performed on 60 cases of giant cell tumor of bone and the results were correlated with the clinicopathologic features. Tumors studied were from 31 men and 29 women whose ages ranged from 18 to 62 years (median, 29 years). The most common sites were the distal end of the femur and proximal end of the tibia, accounting for 75% of the lesions. Treatment consisted of resection in 29 patients (48%), curettage with bone chip packing in 15 patients (25%), or curettage with cement packing in 16 patients (27%). Ten patients (17%) had local relapse within 1 to 3 years, and two had lung metastases. Forty-two patients (70%) exhibited tumors with a diploid DNA content, 16 aneuploid (27%), and two tetraploid (3%). Six (37.5%) of the aneuploid patients had relapses: one of those had been treated by resection of the tumor and five by curettage. Of the remaining ten (62.5%) unrelapsed aneuploid patients, nine had been treated by resection of the tumor and one by curettage. Four of the 42 diploid patients (9.5%) had relapses; all had been treated by curettage of the tumor. The two tetraploid tumors were treated by resection and none relapsed. Histologic parameters did not correlate with relapse rate or DNA pattern. Although relapse was more common among aneuploid tumors, our study shows that this appears to be influenced by the treatment modality rather than the ploidy status. Based on this study the DNA analysis of giant cell tumor of bone has a limited utility for predicting the tumor's biologic behavior.