In the present study, a comparison was made of the fatty acid composition of the grey and white matter of the frontal, parietal and parahippocampal regions of post-mortem brains of patients who had died with Alzheimer's disease (n = 15) and control postmortem subjects (n = 10). Diagnosis of Alzheimer-type disease was based on the presence of senile plaques and neurofibrillary tangles in post-mortem sections. Several highly significant and specific differences were observed between the two groups. Adrenic acid (22:4 n-6) was three to four times higher in the grey matter but lower in the white matter in each of the three regions in the Alzheimer brains than in the control group. These alterations were compensated by reciprocal changes in 18:0 in the grey matter and 16:1 fatty acids in the white matter. There was no significant difference in the proportion of other fatty acids, including those of the n-6 and n-3 series, in either the grey or the white matter of any of the three regions of the two groups, except for a higher proportion of 22:6 n-3 in the parietal white matter in the Alzheimer patients. There was no significant relationship between the levels of the individual fatty acids and age at death. It is suggested that the alterations in the fatty acid composition observed in the brains of Alzheimer patients may be caused by an aberration in the system by which essential fatty acids are transported into the brain.