Normal gender phenotype is determined by normal number and structure of sex chromosomes: XX for females and XY for males. Abnormalities in the number or the structure of sex chromosomes are sometimes associated with a wide spectrum of phenotypes, ranging from almost male through mixed gonadal dysgenesis to female with Turner syndrome [1]. The most common structural abnormalities of the human Y chromosome are dicentrics [1]. Patients with ambiguous genitalia and patients with Turner syndrome, beside a 45,X cell line, frequently have an additional one that contains structural abnormalities of the Y chromosome [2-4]. While the phenotypic gender can be influenced by the 45,X cell line, 4.0-6.2% of female Turner syndrome patients exhibit Y chromosome mosa­icism [5-11]. Phenotypic gender strongly depends on the percentage and distribution of the Y chromosome in the gonads, but in the other tissues mosaicism degree is variable [4,7,11]. However, studies on gonadal tissue are rare­ly available for analysis and alternative, more easily accessible tissue is usually studied. On the other hand, structure of the rearranged Y chromosomes plays an indirect role in the phenotypic gender: since in the idicYp the very proximal breakpoint in the q arm makes the chromosome more unstable; this results in a higher percentage of 45,X cell line, leading to a female phenotype [8,12]. The idicY chromosome instability may arise from loss of the region containing repetitive sequences at the euchromatin/hetero­chromatin boundary of their Y chromosome long arm that has a very important stabilizing role [13]. If the breakpoint position is more proximal in the long arm, then the idicYp chromosome instability is greater, the percentage of 45,X cells is higher and gender phenotype is more likely to be female [14]. We here report on cytogenetic and molecular studies of two isodicentric Y [idic(Y)] chromosomes identified in two patients with ambiguous genitalia and features of Turner syndrome who have mosaic karyotypes.