Summary The relationship between Alzheimer disease (AD) and dementia with Lewy bodies (senile dementia Lewy body type, or SDLT) and dementia in Parkinson's disease is unclear. AD pathology is characterised by both amyloid deposition and abnormal phosphorylation of tau in paired helical filaments (PHF-$tU). In AD, abnormally phosphorylated PHF-$tU is present in neurofibrillary tangles, in neuritic processes of senile plaques, and also in neuropil threads dispersed throughout the cerebral cortex. Cortical homogenates from 12 cases each of AD and SDLT, 13 cases of Parkinson's disease, and 11 normal controls were examined by Western blot analysis with antibodies that detect PHF-$tU. No PHF-$tU was found in Parkinson's disease or control cortex. No PHF-$tU was found in SDLT cases without histological evidence of tangles. PHF-$tU was detectable in SDLT cases with a low density of tangles, and large amounts of PHF-$tU were present in AD cases. This study demonstrates that abnormally phosphorylated PHF-$tU is only present where tangles are found and not in SDLT cases without tangles or with only occasional tangles. It is concluded that Lewy body dementias are distinct at a molecular level from AD.