The presence of Y chromosome material in patients with Turner syndrome is a risk factor for the development of gonadoblastoma. However, no cases with gonadoblastoma or other ovarian malignancies have been found in epidemiological studies of cancer, morbidity, or mortality in Turner syndrome. We examined 114 females with Turner syndrome for the presence of Y chromosome material by PCR. Initially, five different primer sets were used. Y Chromosome-positive individuals were further examined with an additional four primer sets. We found 14 (12.2%; 95% confidence interval, 6.9-19.7%) patients who had Y chromosome material. The karyotype in 7 of these patients did not suggest the presence of Y chromosome material. Seven of the patients had been ovariectomized before entering the study due to verified Y chromosome material, whereas three patients were operated upon after the DNA analysis. The histopathological evaluations showed that 1 of the 10 ovariectomized patients actually had a gonadoblastoma. The rest of the patients did not have gonadoblastoma or carcinoma in situ on histopathological evaluation. Three patients (age, >50 yr) positive for Y chromosome material chose not to have ovariectomy performed, and detailed ultrasonographies did not suggest the presence of gonadoblastoma. The frequency of Y chromosome material is high in Turner syndrome (12.2%), but the occurrence of gonadoblastoma among Y-positive patients seems to be low (7-10%), and the risk may have been overestimated in previous studies, perhaps due to problems with selection bias. This study emphasizes the need for prospective unbiased studies.