Chondroblastoma is a rare, benign primary bone tumor usually involving secondary centers of enchondral ossification. The consistent epiphyseal location in the great majority of chondroblastomas signifies that the tumor may arise from an aberrant germ cell of the physeal plate. This case report describes a chondroblastoma located in a atypical nonphyseal location, namely in the right fourth metatarsal base. Cases such as these imply that the cell of origin may not exclusively be derived form the physeal plate. The clinical-radiographic significance is that historically chondroblastoma is considered an epiphyseal lesion, but rare cases such as the one reported here and those of the skull demonstrate that this is not always a characteristic of these tumors. The implication from a histogenetic standpoint is that chondroblastoma may, on rare occasions, occur in an area of an enchondrally formed bone other than adjacent to the physeal plate. We are presently investigating the histogenetic relationship of the chondroblastoma (CB) of bone to CB of soft tissue, giant-cell tumor of tendon sheath (GCT-TS), pigmented villonodular synovitis (PVNS) and chondroma of tendon sheath (chondroma of soft parts). We now have collected about 15 cases of GCT-TS and PVNS with extensive areas of chondroid and/or cartilage differentiation that cannot be distinguished from CB of bone by histologic or electron microscopic features alone. From these interesting observations we are developing the histogenetic concept that all of these lesions are interrelated to multipotential mesenchymal and/or synovial cells of the tendon sheath.(ABSTRACT TRUNCATED AT 250 WORDS)