Clonal karyotypic alterations of chromosome 6 in three uterine smooth muscle tumors are reported. In all cases an apparently identical breakpoint on the short arm of chromosome 6 was found. Two cases displayed the histologic features of cell-rich myomas with severe nuclear atypia but no clear evidence for malignancy. The remaining case was a primary uterine leiomyosarcoma of an 80-year-old patient showing an apparently balanced reciprocal chromosomal translocation, t(1;6)(p32-33;p21.3), as the sole karyotypic abnormality. This type of aberration has not been reported before in leiomyosarcomas. Because of the nuclear atypia in the other myomas with a breakpoint involving the short arm of chromosome 6 we feel that this cytogenetically recognizable but rare subgroup of uterine smooth muscle tumors warrants a careful clinical follow-up.