Aims: We evaluated the results of molecular diagnostics to see if they can help in conþrming an accurate diagnosis quickly in cases of Ewingñs sarcomas. Methods: We did biopsies and genetic studies in 19 patients (8 females, 11 males Ð 35±19 years old) with a bone tumor with clinical and imaging signs of Ewingñs sarcoma. Cytogenetic examination aimed at tracing characteristic products of the hybrid genes of the tumor. We did molecular analysis with RT-PCR. Results: In ten patients biopsy conþrmed the diagnosis of Ewingñs sarcoma. The genetic tests of 12 patients came to a clear conclusion. In 7 cases (4 Ewingñs sarcomas histologicaly) we had no answer. In seven cases we found products of hybrid genes Ews/Fli and Ews/Erg. These are the result of fusion of genes from chromosomes 22q12, 11q24 and 21q22 and the characteristic chromosomal translocation of Ewingñs sarcoma between exon 7 and exon 6 of the Fli fusion gene was conþrmed. Five cases had no characteristic numerical or structural chromosomal abnormalities. Histologic and cytogenetic diagnoses of Ewingñs sarcoma concur in þve cases. One case of Ewingñs sarcoma was not conþrmed with genetic diagnostics. Two cases with gene mutations characteris tic of Ewingñs sarcoma had an histologic diagnosis of an osteosarcoma. Conclusions: Malignant cells commonly exhibit speciþc chromosomal deletions, which may lead to tumor formation. Our cases show the strong relation between Ewingñs sarcoma and certain chimerical genetic transcriptions. Identical cytogenetical translocations in a few cases of other tumors and their absence in some Ewingñs sarcomas is confusing and indicates their common origin from a primitive tumor.