The clinical, histologic, and histomorphometric features of 42 giant cell tumors (GCT) of long bones and 49 central giant cell granulomas (CGCG) of the jaws were compared. These findings were also correlated with the clinical behavior of 25 cases of CGCG for which follow-up information was available. There was a female predilection for both lesions. The mean ages of patients with CGCG and GCT were 21 and 25 years, respectively. In contrast to CGCG, GCT rarely occurred in persons below the age of 10 years. The only statistically significant quantitative difference between the lesions at the histologic level was the greater number of nuclei in the giant cells of the GCT. There were four significant histologic differences between the two lesions, but 26% of the GCTs were histologically similar to most of the CGCGs and 10% of the CGCGs were histologically similar to most of the GCTs. Five of the 25 patients with CGCG for whom follow-up information was available had recurrences. The average age of those five patients was 11 years, compared to 29 years for those patients without recurrence. All five patients with recurrence were under 17 years of age, and they constituted 45% of the patients in this age group with follow-up. There were no quantitative or histologic differences between the recurrent and nonrecurrent CGCGs that were useful in predicting the likelihood of recurrence. Our findings suggest that the GCT and the CGCG represent a spectrum of a single disease process modified by the age of the patient and the site of occurrence.