Short-term cultures from seven soft tissue leiomyosarcomas were investigated cytogenetically. Sufficient mitoses for chromosome analysis were obtained in six cases, four of which had only normal karyotypes. In one tumor, an intramuscular leiomyosarcoma of the lower arm, a variety of nonclonal structural and numerical aberrations were found in two thirds of the metaphases. Another tumor, a subcutaneous leiomyosarcoma of the knee, had clonal abnormalities resulting in the karyotype 46,X,der(X)t(X;4)(:Xq26----cen----Xp22::4q23----4qter) , del(4)(q23)/47,X,der(X)t(X;4),del(4)(q23), + 20. Flow cytofluorometric measurements of the DNA content in the six leiomyosarcomas successfully karyotyped revealed diploid values in five tumors. The leiomyosarcoma displaying numerous nonclonal changes had two cytofluorometric peaks, 1.01 and 1.39, indicating that the metaphases available for cytogenetic study cannot have been fully representative of the tumor stemline.