I give the details of five cases of Idiocy where the observations were completed by an examination after death. The greatest advantage of such studies as can be made in an Institution for the Training of Idiots is the careful analysis of the mental symptoms, and for this I am much indebted to the teachers for their patient attention and intelligent remarks. The absence of microscopical observations in all the pathological descriptions, save one, is a source of regret to me, though I have been so fortunate as to obtain the report of so competent an observer as Dr. J. Batty Tuke in the case of K. I. The object kept in view in reporting these cases is to throw as much light as possible on the relation of the mental deficiency to the pathological lesions. It is not, therefore, to be expected that they should be reported in the same form as clinical cases published with the intention of illustrating the treatment of ordinary diseases, or the action of new remedies. It is true that the existence of idiocy often modifies the symptoms of ordinary disease, and requires a corresponding modification in treatment; but it would unduly complicate our reports, and probably lengthen the paper to a tedious degree, were commentaries of this kind introduced.