Intraosseous phalangeal growth of giant cell tumor of the tendon sheath (GCTTS) is an uncommon manifestation of this tumor, but it is well known because of its potentially aggressive local growth. This article details the findings of 2 cases of previously unreported intraosseous metacarpal involvement of GCTTS. Differential diagnosis of a metacarpal lesion with decreased radiodensity on radiographs should include GCTTS.