Aim: To observe the characters of 18F-FDG PET in mild Alzheimer disease and compare it with 99Tc-ECD SPECT CBF imaging. Methods: A total of 30 patients with Alzheimer disease treated in the Department of Neurology of Xuanwu Hospital, Capital University of Medical Sciences from April 2003 to May 2004 were selected. Among the 30 patients, there were 15 patients with mild Alzheimer disease and 15 with moderate to severe Alzheimer disease. Meanwhile, 7 healthy persons and 3 spouses of patients with Alzheimer disease were selected as control group. All participants received static PET scanning. Nine normal subjects and 9 mild Alzheimer disease patients further underwent 99mTc-ECD SPECT within a week. Ratio of metabolism and blood flow as a semiquantitative index were calculated by using computer-interested area softwars. Results: According to intention-to-treat, all participants were involved in the result analysis. 1 The ratio of cerebral metabolism in mild Alzheimer disease group (left parietal lobe: 1.08 ± 0.02, left temporal lobe: 1.04 ± 0.06, right parietal lobe: 1.09 ± 0.06, right temporal lobe: 1.07 ± 0.07) and moderate to severe Alzheimer disease group (left parietal lobe: 0.93 ± 0.05, left temporal lobe: 0.98 ± 0.05, left frontal lobe: 1.15 ± 0.10, right parietal lobe: 0.95 ± 0.05, right temporal lobe: 1.01 ± 0.09, right frontal lobe: 1.05 ± 0.08) was significantly lower as compared with that in control group (left parietal lobe: 1.21 ± 0.15, left temporal lobe: 1.19 ± 0.11, left frontal lobe: 1.31 ± 0.13, right parietal lobe: 1.25 ± 0.12, right temporal lobe: 1.22 ± 0.10, right frontal lobe: 1.34 ± 0.14, t = 2.04 - 5.71, P < 0.05 - 0.01). 2 Rate of cerebral blood flow in mild Alzheimer disease group (left parietal lobe: 0.81 ± 0.04, left temporal lobe: 0.79 ± 0.12, right parietal lobe: 0.86 ± 0.10, right temporal lobe: 0.83 ± 0.08) was significantly lower as compared with that in control group(0.90 ± 0.06, 0.89 ± 0.09, 0.87 ± 0.07, 0.93 ± 0.08, 0.91 ± 0.7, 0.92 ± 0.08, t = 2.08 - 2.82, P < 0.05). 3 Twelve of 15 moderate to severe Alzheimer disease patients indicated the decreased intape of symmetry and radioactivity in bilateral parietal lobes and temporoparietal lobes [80% (12/50)] showed by PET, and 3 cases indicated decreased radioactivity in unilateral temporal lobe[20% (3/15)]. Thirteen of 15 mild Alzheimer disease patients indicated the decreased intape of symmetry and radioactivity in unilateral parietal lobe, temporal lobe and temporoparietal lobe[87% (13/15)] showed by PET, adn 2 cases indicated decreased radioactivity in bilateral parietal lobes [13% (2/15)]. Six of 9 mild Alzheimer disease patients indicated the decreased unilateral intape of radioactivity showed by SPECT, and no positive expression was showed in another 3 patients. Conclusion: Patients with mild Alzheimer disease indicate both decreased cerebral metabolism and blood flow. The glucose hypometabolism in mild Alzheimer disease is characterized with onset in unilateral parietal lobes, temporal lobes or temporoparietal lobes. The cerebral blood characters are similar to metabolism characters. But the sensitivity of PET is lower than that of SPECT. SPECT brain perfusion imaging is considered as a screening method for mild Alzheimer disease detection, and PET is the best choice for the patient suspected of mild Alzheimer disease, especially when SPECT is negative.