Androgen excess, found occasionally in gonadal dysgenesis, may be associated with hilus cell hyperplasia within the gonadal streaks. This report describes a phenotypic female with Turner syndrome, clitoromegaly, hilus cell hyperplasia, and a gonadal-to-peripheral venous step-down gradient for testosterone, androstenedione, and estradiol. Cytogenetic analysis of cells from peripheral blood and both streak gonads revealed a mosaic karyotype, 45,X/46,X,iso(X). The isochromosome was a nonfused dicentric type (pter leads to q2105 leads to pter). Despite absence of the Y chromosome, serologic analysis revealed the presence of H-Y antigen, thereby suggesting occult Y chromosomal function in the patient's gonads.