We describe a 17-year-old girl with hypersexuality resulting from virilization, the latter a consequence of polycystic ovary syndrome, and we review the literature pertinent to hypersexuality in children and adults. Inappropriate sexual behavior (a common cause of disruption among children who are hospitalized for psychiatric disorders) may be caused either by hypersexuality or by simply ill-regulated behavior: a definition of hypersexuality is proposed that can be applied at the bedside (namely, sexual behaviors or fantasies that have abruptly increased in frequency by comparison with a previous baseline, are of sufficient excessiveness to disrupt expected or usual social, academic, or occupational functioning, or constitute a source of distress), so that this distinction can be made promptly, and we present a differential diagnosis for hypersexuality to direct its evaluation. Virilization does not seem to be a common cause of hypersexuality in children and adolescents, but it should not be overlooked when it does exist. The differential diagnosis of hypersexuality in adults, which we present for comparison, is much larger than it is in children.