Dr F. Magnier, in his Thèse de Paris, 1900, shows that apart from idiocy and imbecility there are well-marked and characteristic mental diseases affecting the young before adult life is reached. Among these affections dementia præcox, as already shown in these columns (The Lancet, February 10, 1900, p. 397) holds a prominent place. The others include acute mental confusion, acute delirium, folie périodique, menstrual psychoses, and paranoia. As regards acute mental confusion it is pointed out that whereas Meynert considered the age of election of this disease to be between 20 and 30 years, several cases have been recorded of its occurrence earlier. Thus Trenel has recorded its occurrence at nine years of age in a girl, of alcoholic and neurotic parentage, consequent upon fright, and Mannheimer has recorded other cases in girls below the age of 20 years. Acute delirium with high temperature and adynamia ending fatally is recorded in two cases by Dr Magnier, both of which occurred in females of the age of 19 years and 26 years respectively. Periodic insanity in 80 per cent of cases begins before the age of 25 years, the depressive form occurring especially in the decade between 20 and 30 years of age. Dr Magnier points out that an insane heredity is present in 80 per cent of the cases, and he cites the case of a girl with periodic insanity which had its onset at the age of 14 years and was marked by agitation, insomnia, and mutism, while in a second case the disease began at the age of 12 years and was marked by suicidal impulses. Two other cases commenced at the age of 18 years and one at the age of 29 years and a month after parturition. The menstrual psychoses are characterised by agitation, loquacity (singing and talking incessantly), and hallucinations of sight and hearing. They occur during menstruation and subside soon after and recur with the menses. The clinical type is that of adolescent insanity. Though paranoia is a disease of adult life yet transitory excitement may occur in youth from time to time as a prodrome. Thus in one case symptoms of suspicion and auditory hallucination began at the age of 22 years and three years later the delusions became well marked. Another commenced at the age of 21 years. From the facts and cases recorded in Dr Magnier's Thèse it thus appears that the period of youth and adolescence is liable to develop a variety of mental affections.