Semantic dementia is a syndrome of progressive deterioration in semantic memory (knowledge of objects, people, concepts and words). It falls within the clinical spectrum of frontotemporal dementia but its pathology is yet to be studied systematically. This study included 18 consecutive post mortem cases meeting clinical criteria for semantic dementia. Clinic records and diagnostic histopathology were available for all cases; structural neuroimaging, neuropsychology and semi-quantitative histopathology/immunohistochemistry data were analysed where possible. The pathological diagnosis in a clear majority of cases was frontotemporal degeneration with ubiquitin inclusions (n = 13). Eleven of these cases had characteristic motor neuron disease-type inclusions in the dentate gyrus and cerebral cortex. Ubiquitin inclusions were found only in the inferior olivary nucleus in the other two, one of which was the only case to show degeneration of motor tracts and also to have shown evidence of motor neuron disease during life. None of the patients had motor symptoms or signs at presentation. A family history of motor neuron disease was documented in one case. Pick body-positive Pick's disease appeared three times. Two cases had Alzheimer's disease and significant coincidental Alzheimer-type pathology was also found in one of the ubiquitin inclusion cases. One of the Alzheimer's disease patients had changes in white matter signal on scanning, whereas all other scans showed cerebral atrophy only. Semi-quantitative assessment of regional neuronal loss found that anterior and inferior temporal regions bore the brunt of disease across all histopathological subtypes, usually on the left side, implicating this region in semantic processing.