In order to evaluate the relationship between perfusion brain SPECT and specific cognitive functions as well as the possible influence of the illness severity on them, 34 patients clinically diagnosed as probable Alzheimer disease (AD) and 12 elderly controls were studied. AD patients were subdivided according to severity as 16 mild AD and 18 moderate AD. Neuropsychological battery of CERAD protocol and a semiquantitative analysis of 99mTc-HMPAO-SPECT images was carried out in all groups. With regard to the regions affected in SPECT, involvement of temporo-parietal cortex were of most use in discriminating between AD patients and controls, but only temporal hipoperfusion distinguished mild disease from controls. Memory and praxis impairment correlated with temporo-parietal perfusion. Frontal involvement seemed to be a discriminator of disease progression, nevertheless, a significant correlation was present between memory and frontal hipoperfusion indicating the diffuse and sometimes heterogeneous distribution of AD pathology.