Diffuse tenosynovial giant cell tumor (DGCT) could present as a large intra-articular mass (pigmented villonodular synovitis, or PVNS) or as an extraarticular mass, which might be confused with a sarcoma, particularly when growth is destructive and giant cells are few. Prompted by a DGCT in which a subpopulation of cells was desmin positive and in which an erroneous diagnosis of myosarcoma was made, we analyzed the frequency of desmin, myogenin, MyoD1, and muscle-specific actin immunoreactivity in 45 well-characterized GCTs. We also analyzed a subset of these cases with antibodies to smooth muscle actin, as well as macrophage, follicular dendritic cell, extrafollicular dendritic cell, and dermal dendrocyte-associated antigens. Sections from 45 cases of formalin-fixed GCTs (22 DGCTs, 13 cases of PVNS, and 10 localized GCTs) were immunostained for desmin, myogenin, MyoD1, and muscle-specific actin. The eight cases that showed the largest number of desmin-positive cells were immunostained for smooth muscle actin, CD45, CD68, CD21, CD35, cytokeratin 8, and Factor XIIIa. Desmin-positive cells were seen in 20 (43%) of 45 cases: 10 (43%) of 22 DGCTs, 5 (38%) of 13 cases of PVNS, and 5 (50%) of 10 localized GCTs. In contrast, none were positive for any of the other muscle-associated proteins. In almost all of the cases, the desmin-positive cells were large and dendriform, with long processes that interdigitated between adjacent round cells. Desmin immunoreactivity was found in almost 50% of all GCTs, in the absence of positivity for other muscle markers. Desmin immunoreactivity in GCT seemed to be confined to a variably sized subpopulation of large dendritic cells whose exact identity remains uncertain.