A patient with a mitotically unstable dic(Y)(p11) chromosome is reported. Physical examination revealed a small penis with severe hypospadia, undescended testes, rudimetary vagina, uterus, left fallopian tube, and no stigmata of Turner syndrome. Longitudinal chromosome studies over a four-year period, including blood, skin, foreskin, and testicular tissue, revealed 45,X/46,X,dic(Y)(p11)/46,X,del(dic Y) mosaicism. The proportions of these cells varied in the different tissues, and only 45,X and 46,X,del(dic Y) were major cell lines in testicular tissue. Additional minor cell lines were present mainly in peripheral blood: 47,X,dicY,dicY; 47,X,dicY,del(dicY); and 47,X,del(dicY),del(DICY). Premature disjunction of one of the centromeres in a high percentage of the dicentric Y chromosomes in metaphase was observed by Q- and C-banding. Lymphocytes at anaphase and telophase showed lagging Y chromosomes, fragments, and nondisjunction. These observations indicate a high degree of mitotic instability and thus raise the question of the effect of premature centromeric disjunction on mitotic instability of dicentric chromosomes.