Mutations in the luteinizing hormone (LH) receptor gene have been found in patients with abnormalities in their sexual differentiation. In this paper we review results obtained in the studies of these LH receptor mutations. Activating and inactivating mutations are discussed with respect to the mechanism of action of LH/human chorionic gonadotrophin but also in light of their impact of the present knowledge of the physiology of sex differentiation and gonadal function.