Immunocytochemical studies were carried out on two previously reported autopsy cases of Lhermitte-Duclos disease. The unaffected cerebellar cortex adjacent to the lesions served as control. The findings supported the view, previously expressed by one of the authors, of a heterogeneous neuronal structure of the lesion, consisting of at least two cell types. No further light was thrown on the predominant medium-sized cells, believed to represent hypertrophic internal granular neurons. On the other hand the large cells shared a number of features with Purkinje cells. In particular they were recognized by the pan-T-cell antibody anti-Leu-4, were surrounded by axosomatic synapses visualized by the antisynaptic vesicle glycoprotein antibody SV2, and contained both non-phosphorylated and phosphorylated neurofilament epitopes. It is suggested that these cells represent dysplastic Purkinje cells. The lesion therefore appears to be a complex hamartoma rather than a simple hypertrophy of the internal granular neurons.