Chromosomal observations by banding technique in 18 short-term cultured human uterine leiomyomas are reported. Half of the tumors had a primary or secondary abnormal stemline. They were usually characterized only by structural changes, in particular reciprocal translocations or insertions. Reviewing already published cases together with the new material confirmed that the aberrations in abnormal stemlines predominantly affected chromosomes 1, 2, 6, 7, 12, 14, and X. In these chromosomes the regions 1p36, 2p24, 6p12-21, 7q21-31, 12q13-15, 14q22-24, and the short arm of the X chromosome were preferentially affected. As in two other thoroughly studied human benign tumors, the pleomorphic adenoma and the meningioma, the very specific but sometimes complex chromosomal aberrations in leiomyomas could well be events of primary evolutionary importance. Likewise, in cases with a normal stemline, it is possible that comparable changes in the corresponding specific chromosomal regions have occurred at a submicroscopic level. Ascertaining this possibility, as well as the role of the aberrations with regard to the benign nature of the tumors, must be the focus of future analysis using molecular techniques.