Objective To investigate the plasma levels of endogenous hydrogen sulfide (H2S), a gasotransmitter, and homocysteine (Hcy) in patients with Alzheimer' s disease ( AD), vascular dementia ( VD), and cerebrovascular disease (CVD) and their role in the pathogenesis of AD and VD. Methods 31 AD patients, 28 VD patients, 20 CVD patients , and 23 normal controls ( NC group) underwent examination of plasma concentrations of H2S and Hcy, mini mental state examination (MMSE), activity of daily living scale (ADL), Hachinski ischemic score (HIS) , and Hamilton's depression scale (HRSD). The severity of AD and VD was evaluated according to the global deterioration scale (GDS). Results (1) The plasma H2S levels of the AD, VD, and CVD patients were (34 ± 7), (36±5), and (37 ± 7) μmol/L respectively, all significantly lower than that of the NC group [ (45±7) μmol/L,all P <0. 01 ]. However, there were no significant differences in the plasma H2S level among the AD, VD, and CVD patients (all P 0. 05). The plasma H2S concentration was negatively correlated the severity of AD. (2)The plasma Hcy levels of the AD, VD, and CVD patients were ( 14. 0 ± 3. 0), ( 16. 0 ± 6. 1 ), and ( 14. 4 ± 4. 9 ) μmol/L respectively, all significantly higher than that of the NC group [ (9.8±2.5) μmoL/L,P <0. 01 ). However, there was no significant difference in the plasma Hcy level among these 3 groups ( all P 0. 05). The plasma Hcy concentration was positively correlated with the severity of AD or VD. Conclusion H2S and Hcy are both involved in the pathogenesis of AD, VD, and CVD, and its alteration in level may be associated with the severity of AD and VD.