This child developed signs of a serious mental illness during an attack of jaundice three years ago, just before his 11th birthday, and the condition has continued to develop up to the present time, though not quite evenly. At one period the illness was thought to have remitted, but it is evident in retrospect that the remission was, in fact, characterized by regression in behaviour and considerable invalidism. The clinical picture showed sustained mutism and one postural oddity as the most prominent features; and in other respects the child showed an interest in his environment and participation in outside activities which in an adult with a similar diagnosis would be regarded as unusual. There was a morbid heredity in the case and a personality of distinctly pathological character which was clearly shown by his behaviour during his early years. The genesis of the reaction was complicated by an acute infective illness and by two head injuries, neither of which now appears to have been of importance, but which led to much clinical obscurity at the time. The anomalous clinical pattern and the inability of the child to communicate his experiences illustrate the difficulties of diagnosis at this age, and an attempt is made here to outline other possibilities which need consideration and the reasons in this case for rejecting them.