After a hiatus of some decades, gender transition was recognised and regulated by the legal and medical professions in Japan from the mid-1990s. Since the first of these gender reassignment procedures were carried out, the courts and the legislature have repeatedly had to come to terms with new situations. Decisions have been made on such matters as changing an individual’s gender on their family register and other official documents, recognition of a transgendered person’s marriage and, most recently, how to register the child of a marriage involving a transgendered parent. Assisted reproductive technologies have complicated this picture. Although there are gaps between the legal and medical discourses on gender transition, both the medical and legal professions operate according to assumptions about what constitutes a proper family. There are ever-widening gaps between the actual forms and practices of families in Japan, the cultural representations of alternative family forms, and the official policies of the legal and medical establishments.