Quantitative morphometric (stereological) methods have been used to assess the effects of accumulation of neurofibrillary material on the fine structure of pyramidal cells in biopsy specimens of temporal cortex from nine patients with Alzheimer's disease. When compared with non-tangled cells from the same patients, tangled cells show an increase in total area of cytoplasm due to the accumulation of tangle and a reduction in the area of the nucleus; the area proportion of the cell body occupied by total cytoplasm, therefore, increases whereas that of the nucleus decreases. Within the total cytoplasm, nucleolar and mitochondrial areas are maintained, but that of lipofuscin is increased, though all are increased when expressed as a proportion of the useful cytoplasm alone (i.e. total cytoplasmic area minus area occupied by tangle). Measures of the amount of rough endoplasmic reticulum and ribosomes are decreased overall in tangled cells, though when related to useful cytoplasm alone such measures approach non-tangled cell values. Measures of smooth endoplasmic reticulum are unaltered throughout. When related to the amount of tangle within cells it was found that the most heavily tangled cells retain 28% of useful cytoplasm, 72% of the nuclear area, 50% of the rough endoplasmic reticulum and 27% of ribosomes present within least tangled and non-tangled cells. By contrast, mitochondrial area is maintained and that of lipofuscin increased. The capacity for protein synthesis in tangled cells appears, therefore, to be progressively decreased with accumulation of tangle, whereas that for oxidative metabolism is maintained and lysosomal activity, perhaps, increased. Neurofibrillary tangle formation and accumulation may, therefore, lead to the eventual death of neurons and be the major cause of nerve cell loss in Alzheimer's disease.