A 46 XY individual with male pseudohermaphroditism was investigated. The phenotype was distinctive in that the habitus was female in character, but partial fusion of the labioscrotal folds, testes, and male wolffian duct structures that terminated in the vagina were present. Müllerian structures were absent. At the expected time of puberty both feminization (breast development) and virilization (clitoral enlargement) took place. Studies of estrogen and androgen dynamics revealed plasma testosterone levels and production rates characteristic of normal men. Plasma estrogen levels and production rates were greater than those of normal men. Plasma gonadotropin levels were also high. These findings suggest that the fundamental defect in this patient is androgen resistance rather than defective androgen synthesis. Dihydrotestosterone formation from testosterone slices of epididymis and perineal skin was normal. The family history was uninformative. On endocrinologic, genetic, and phenotypic grounds the syndrome of incomplete testicular feminization can be separated from the complete form of testicular feminization and from familial incomplete male pseudohermaphroditism, Type 2. Additional studies will be required to determine whether this disorder is also distinct from the Type 1 form of familial incomplete male pseudohermaphromditism.