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-follicular-phase 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 (n=71). RESULTS: In group 45,X/46,XX, 8% or more of aneuploidy accounted for a smaller height compared to controls (P=0.01). Body mass index was increased from 6% of aneuploidy (P=0.02) and was positively correlated to the percentage of 45,X cells (P=0.0001); menarche occurred earlier from 10% of aneuploidy (P=0.01) and was inversely correlated to the percentage of 45,X cells (P=0.045). 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 (P=0.01), and recurrence was positively correlated to the percentage of aneuploidy (P=0.008). CONCLUSION: Sex chromosome mosaicism is responsible for clinical changes from 6% of aneuploidy, corresponding to the main phenotypical features of TS.