Numerical and structural chromosomal aberrations were found in metaphases from lymphocyte cultures from Sardinian patients with the sporadic type of Kaposi sarcoma, and also in fibroblasts derived from cultured biopsies of the tumoral lesions. In several cases there was evidence of clonal evolution of some of the chromosomal aberrations. The chromosomes most frequently involved in numerical aberrations were 10, 13, 15, 22 and the X and Y chromosomes, and those most frequently involved in translocations and deletions were 7, 13, 15, 22 and the X chromosomes. The hypothesis is made that in Kaposi sarcoma the chromosomal instability and clonal evolution in vivo could be modulated by the immunologic situation peculiar to the condition.