Cytogenetic investigations of two thymomas of different histopathology revealed unrelated clonal karyotypic changes: 44,XY,+X,inv(2)(p25q13),del(6)(q15),-8,-16,-17 in a cortical thymoma, and 48-49,XX,+del(X)(q24),+i(5p),+?del(7)(q22),der(11)t(1;11)(q23;q25),t(11; ?)(p15;?),-18,+r in a mixed-type thymoma. These are the first thymic tumors of epithelial origin that have been investigated with banding technique.