The authors described three cases of intraabdominal desmoplastic small round cell tumour of the peritoneum (IDSRT). In one case the patient was a woman, and in the other two men. The age ranged from 20–29 years. Common of all the cases was a rapid onset of clinical symptoms during the period of twelve to eighteen months. In one case, a 22-year-old woman presented with a symptomless course of disease documented by medical examination one month ago. Intensive chemotherapy was applied but two patients died of generalisation. The 22-year-old woman is alive but with clinical evidence of generalisation in the abdominal cavity. The „classical“ type of IDSRT was found in all the cases. Sharply demarcated groups of tumour cells of different size were surrounded by dense fibrous stroma. In some regions desmoplastic areas prevailed. In one case the tumour consisted of round and oval cells resembling a lymphoma. In the other two cases, the slightly elongated cells were present. Immunohistologically, the small round cells were positive for cytokeratins with antibody AE1-AE3. Membrane and dot-like paranuclear positivity were found. In 2 cases the reaction to desmin was seen in a dot-like paranuclear distribution, whereas the reaction to smooth muscle actin (MSA) was negative. In all the cases positivity to vimentin and neuron specific enolase (NSE) were apparent. Negative reactions were found for WT-1 antibody in all three cases. In one of the cases the RT PCR reaction for chimeric gene EWS/WT1 was performed, and found to be negative. Many different tumour types, such as lymphoma, Ewing sarcoma/PNET, neuroblastoma, alveolar rhabdomyosarcoma, malignant mesothelioma must be excluded. Cytogenetic examination should be performed on tumours with a „non-typical“ histological pattern and uncommon inmmunohistological examinations.

        Key words: intraabdominal desmoplastic small round cell tumour of the peritoneum – immunohistological findings – electron microscopic findings