A detailed banded chromosome analysis was performed in five established Ewing's sarcoma (ES) cell lines originating from four unrelated patients in relapse. Of various numerical and structural abnormalities, a reciprocal translocation between chromosomes #11 and #22, t(11;22)(q24;q12), was observed in four of the lines. The t(11;22) was seen in every cell in three lines; in the fourth, it was seen in only 21% of the cells considered stemline, but the der(22) was present in the remaining 79% of cells. These results suggest that t(11;22)(q24;q12) is a chromosomal change specific to ES cells, in which the rearrangement of chromosome #22 could be the consistent karyotypic feature and the crucial step in terms of cell proliferation. Other, nonrandom chromosomal changes were found: monosomies 2p11----2pter, 10q25----10qter, and 17pter----17q11, and partial trisomies 1q21----1q31 and 8q24.1----8q24.2. The role of the therapeutic regimen received by these patients must be evaluated with regard to the formation of a wide variety of homogeneously staining regions, which were observed in every cell line, particularly on the short arm of chromosome #7, which was observed in three of the five cell lines.