In early studies of chromosome disorders, affected individuals were characterized by gross morphological changes, mental retardation, and/or impaired sexual development. Later studies examined chromosome disorders' relationship to behavior disturbances. Findings of unexpectedly high incidence of individuals with an extra Y chromosome among inmates of Scottish maximum-security prisons generated both publicity and controversy. Numerous laboratory techniques are available for studying sex chromosome abnormalities. Interest in the extra Y chromosome was stimulated by early case reports from Klinefelter and others, and the incidence of sex chromosome abnormalities has been studied in a variety of populations. Clinical features of the XYY group have included increased stature, abnormal neurological signs, endocrine dysfunction, and a wide variety of psychological abnormalities. For the individuals showing no morphological, neurological, or intellectual abnormalities, several explanations have been advanced for the etiology behavioral changes. Critiques of the research findings have focused on both methodology and on conceptual problems. Literature on legal implications of the XYY syndrome emphasizes philosophical judicial issues and procedural matters relating to the pleas of not guilty. It is concluded that the relationship between XYY chromosome and antisocial behavior has been neither proven nor disproven. Moreover, simple explanations for complex behavior, such as antisocial behavior, should be avoided. The presence of a sex chromosome disorder may in no way dictate either the inevitability of behavior disturbance or its form. Medicine should focus on identifying individuals at risk and providing for their needs. Ten references are included.