Activated forms of microglia were ultrastructurally evaluated in three neurological diseases of different aetiology (subacute sclerosing panencephalitis--SSPE, Wilson's disease and Alzheimer's disease). The occurrence of activated rod, ramified and amoeboid microglia was found in the investigated diseases. The widest ultrastructural variability of microglia was in SSPE, including the presence of mitotic chromosomes or centrioles in its cytoplasm, which indicates microglia proliferation. In the nuclei of activated microglia, some nuclear bodies with different structures were frequently seen, whereas lamellar structures (similar to developing Birbeck's bodies--pathognomonic to Langerhans-type dendritic cells) were observed in the cytoplasm. The activated forms of microglia with apoptotic features were found only in SSPE cases. Some apoptotic nuclei were filled with nucleocapsids of measles virus. In Alzheimer's disease, activated microglia was most frequently bound to senile plaques. Ramified microglia was in contact with amyloid fibrils, which penetrated its cytoplasm and reached the nuclear membrane and channels of rough endoplasmic reticulum, or was situated among dystrophic neurites. Rod microglia was found predominantly at the edge of senile plaques. In Wilson's disease, the ultrastructure of activated microglia showed mostly indirect forms between rod, ramified and amoeboid microglia. The microglia ultrastructure suggests that its morphological form may express functional involvement in the pathogenesis of a given disease entity.