Objective: Turner's syndrome (TS) is well known, but prognosis for 45,X/46,XX mosaicism below 30% of aneuploidy has not been established. We evaluated differences in clinical features and biological parameters between patients with numerical sex chromosome mosaicism diagnosed incidentally and control women. Design: Retrospective observational study of clinical features and biological parameters. Methods: Standard endocrinological and gynecological examination was done and early-follicularphase blood values were collected from the medical records of women aged 21-43, who were referred to our ward from 1996 to 2006 because of infertility and were karyotyped. Seventy-one women with sex chromosome mosaicism (45,X/46,XX) ranging from 4 to 28% were assigned a chromosomally normal woman (46,XX) matched according to age (nZ71). Results: In group 45,X/46,XX, 8% or more of aneuploidy accounted for a smaller height compared to controls (PZ0.01). Body mass index was increased from 6% of aneuploidy (PZ0.02) and was positively correlated to the percentage of 45,X cells (PZ0.0001); menarche occurred earlier from 10% of aneuploidy (PZ0.01) and was inversely correlated to the percentage of 45,X cells (PZ0.045). No difference was found between the groups for FSH, LH, estradiol, inhibin B, and TSH values. Spontaneous abortions were more frequent in case of mosaicism (PZ0.01), and recurrence was positively correlated to the percentage of aneuploidy (PZ0.008). Conclusion: Sex chromosome mosaicism is responsible for clinical changes from 6% of aneuploidy, corresponding to the main phenotypical features of TS.