The cytologic findings in nine cases of giant-cell tumor (osteoclastoma) of bone diagnosed by fine needle aspiration (FNA) biopsy are described. The aspirates contained a dual population of cells, consisting of mononucleated tumor cells and multinucleated giant cells (osteoclasts). The tumor cells were usually evident in the FNA smears as cohesive perivascular clusters; some were present as sheets or single cells. The most striking feature, which may best suggest the diagnosis of giant-cell tumor in an FNA smear, was the attachment of the osteoclasts to the cohesive groups of tumor cells. Even when the cells were more dissociated, there was a rather regular distribution of the two types of cells. This series further demonstrates that FNA biopsy permits an accurate cytopathologic diagnosis of giant-cell tumor and that its use can clarify the clinical and radiologic impressions of these bony lesions preoperatively. The differential diagnosis between osteoclastoma and other giant-cell containing bone lesions is discussed in detail.