Matsumoto F, et al. - The medical records of consecutive individuals (n = 34) with 45,X/46,XY karyotype or its variants underwent gonadal biopsy or gonadectomy at a single institute between 1996 and 2017 were retrospectively reconsidered in order to define the gonadal characteristics of individuals with 45,X/46,XY mosaicism, and to estimate the prevalence of gonadal tumor in various phenotypes. A unilateral dysgenetic testis and a contralateral streak gonad were identified in 20 individuals, bilateral streak gonads in nine, and bilateral dysgenetic testes in five. The findings suggested that the prevalence of gonadal tumors in individuals with 45,X/46,XY mosaicism may differ according to the phenotype and raised in individuals with women's phenotype. Thus, considering the heightened risk of gonadal tumors in such people, initial investigation and individual management, involving prophylactic gonadectomy, are suggested. Moreover, in males, a close follow-up of the preserved testes is necessary until adulthood.