This paper is a preliminary report of some investigations we are now making into the pathology of mental deficiency, mainly from the histological point of view. We have so far examined 22 cases, comprising 7 low-grade defectives showing gross neurological lesions, chiefly of extrapyramidal character, 6 microcephalics with spastic diplegia and severe mental defect, 2 able-bodied microcephalics without gross neurological signs, 1 simple able-bodied idiot and 6 mongols. In this short paper it is impossible to give even the slightest indication of the wealth of pathological material found in such a variety of widely differing conditions. A full description of our findings, together with a more detailed discussion, must be left to further papers dealing with individual groups of cases. The purpose of making a short note at this stage is to draw attention to one particular finding which struck us by its surprising constancy, i.e., a proliferation of the fibrous glia, particularly pronounced within the cerebral and often also the cerebellar white matter. In many of the cases the cortical changes were slight compared with the intense lesions of the white matter. In none of them was demyelinization a well-marked feature, nor did its severity approach that of the glial proliferation; in many it was negligible, and there were none of the fatty breakdown products, characteristic of the demyelinizing disease of the white matter called after Schilder. The gliosis was either diffused or patchy, and was often markedly perivascular; even stripe-like scars, as described by Hallervorden (10), were to be seen, although, contrary to this author's observation, there was, as a rule, no corresponding myelin defect.