Background: Alzheimer's disease is the most common form of dementia in the elderly. Serum levels of homocysteine have been related to increased cortical and hippocampal atrophy. We aimed to determine the serum levels of homocysteine, folate, and vitamin B12 in patients with Alzheimer's disease. Methods: Blood levels of homocysteine and its biological determinants, folate, and vitamin B12 were measured in 51 patients who were diagnosed as having Alzheimer's disease according to DSM-IV criteria and compared with the serum levels obtained from 49 control individuals. Results: The mean serum homocysteine concentration was significantly higher in patients with Alzheimer's disease than the controls (20.4 ± 16.5 µmol/L v 14.5 ± 5 µmol/L; P= 0.02). There were no statistically significant differences between the mean serum levels of vitamin B12 (P=0.6) and folate (P= 0.3) in the patients and the controls. There was no correlation between age and serum homocysteine concentration in both groups (P= 0.8). Conclusion: Serum homocysteine concentration was significantly higher in the patients with Alzheimer's disease. This biomarker might be considered as a predictor of cognitive performance.