Representing only about 1% of all primary bone tumors, chondroblastoma constitutes a very rare bone tumor entity. 56 cases of chondroblastoma, that had been collected by the Hamburg Bone Tumor Registry from 1972 to 1995, were examined histologically together with the radiological and clinical findings. In addition immunohistochemistry with antibodies against S 100, PGM1, LCA and the proliferationmarker MIB 1 was performed. The mean age was 20.4 years and male patients being the majority with a gender ratio of 2.7:1. Predominant localisation was the epiphyses of the long bones, although almost 40% of the tumors were located at untypical sites. Usually a well-circumscribed lysis could be seen on plain X-Ray examination, however partial cortical destruction could be observed in one third of the cases. Histologically characteristic was a polygonal cell component with a weblike chonroid matrix, sometimes with a plane-like appearance. 5 cases showed a distinct nuclear polymorphism making a distinction from osteosarcoma difficult. Using immunohistochemistry all tumors except for one showed positive reaction for S 100 protein. Although the histogenesis of chondroblastoma is not completely understood, morphological findings as well as the observed reactivity with the S 100 protein indicate the chondroid origin. No reactivity for PGM 1 (CD 68) or LCA could be detected. All chondroblastoma showed a low rate of proliferation, thereby being distinguishable from high malignant bone tumors. In general chondroblastoma show a benign biological behavior. Different behavior was observed in 2 cases. One relapse located in the pelvis revealed local aggressive growth while in another case in the humerus a malignant transformation had taken place.