Excessive random loss and gain of simple chromosomes at the diploid and tetraploid levels, and numerous chromosomal aberrations, have been observed in lymphocytes and fibroblasts, cultured from Gardner syndrome and familial polyposis coli patients and children at risk for multiple adenomas. This increase in numerical and structural aberrations seems to represent a general instability of chromosomes in the lymphocyte and fibroblast nuclei of patients with multiple adenomas in the colorectum. A consistent heteromorphism of chromosome No. 2 homologous tentatively identified as a deletion was observed in all 17 patients with multiple adenomas in the colorectum, and two younger people of 13 and six years of age who are at risk for Gardner syndrome but do not have colorectal polyps. The chromosome No. 2 aberration was not observed in two symptomatic patients with occasional discrete colorectal adenomas.Eighteen controls who did not have Gardner syndrome nor familial polyposis coli did not show the structural aberration in chromosome No. 2 nor chromosome instability.