The somatic form of CD 143 (angiotensin-I-converting enzyme) has been detected in many endothelial cells of the human body. Recently, we could identify local CD 143 overexpression in various pathological conditions e.g. in capillary endothelial proliferations at the border of myocardial infarctions and in coronary atherosclerotic plaques. Using monoclonal antibodies against CD 143, CD 31 and CD 34 we evaluated, whether CD 143 can serve as a marker of endothelial differentiation in vascular/perivascular tumors. 39 benign and malignant tumors, formalin-fixed, paraffin-embedded were investigated by APAAP immunohistochemistry using epitope retrieval techniques. Expression patterns were separately qualitatively and semiquantitatively analysed. Positive detection of epitopes was registered if more than 10% of the tumor cells were stained. CD 143 was detected in the majority of benign and malignant vascular tumors. All benign and malignant haemangioperizytomas were CD 143 negative. None of the markers detected all haemangioendotheliomas or all angiosarcomas, but the combination of CD 143 and CD 31 turned out to be positive in all haemangioendotheliomas and angiosarcomas. We conclude that most of the benign and malignant endothelial tumors express CD 143. The best discrimination of intermediate grade and malignant endothelial tumors can be achieved by analysis of the CD 143 and CD 31 co-expression.