STUDY OBJECTIVE: To evaluate the prevalence of germ cell tumors and the clinical monitoring practices for those who deferred prophylactic gonadectomy in a large North American cohort of individuals with Turner syndrome with Y-chromosome mosaicism (TS+Y). METHOD: A query of the medical records at multiple North American children's hospitals was done using ICD codes related to Turner Syndrome. A retrospective chart review was conducted on those patients between ages 0 to 30 years with Y-mosaicism. RESULTS: The data of 57 participants were analyzed. Eight (25.8%, n = 31) ≥ 13 years underwent spontaneous thelarche. One (3.2%) had spontaneous menarche. Forty-seven (82.5%) had gonadectomy at a median age of 8 years (IQR 11.0, range <
1 to 19 years). Sixteen (34%) had growth hormone therapy exposure prior to gonadectomy. Fourteen (29.8%) had gonadoblastoma. Two (4.3%) had dysgerminoma. Differences in age at gonadectomy, presence of the entire Y-chromosome, and exposure to growth hormone when comparing those with vs without gonadal tumor were not statistically significant. Gonadectomy had not been performed in 10 individuals, median age 6.5 (IQR 9.0, range <
1 to 14 years). There was no consistency in the plan for ultrasound and/or tumor markers for follow-up. CONCLUSIONS: Our data shows a prevalence of 24.6% of gonadal tumors in individuals with TS +Y and a relatively low risk of malignant transformation (3.5%). Prior exposure to growth hormone was not predictive of the presence of gonadal tumor. Future cytogenetic studies are needed to better understand the factors involved in the development of gonadal tumors.