Aim The aim of the present study was to provide first‐hand information about the prevalence of mild cognitive impairment (MCI) and Alzheimer's disease (AD) in Tamil Nadu, a southern state in India, and examine if there exists a relationship between cognitive functions and biochemical parameters in these patients. Methods Surveys were collected from adults, older men and women (n = 3126) from different regions of Tamil Nadu, which were followed up after 12 months for 1337 participants. Mini‐Mental State Examination (MMSE) scores, lipid profile, and liver function tests were carried out in the elderly, MCI and AD patients. Based on the MMSE scores, the elderly population was classified into old control (28.97 ± 1.49; n = 1868), MCI (19.58 ± 1.17; n = 734) and AD (7.18 ± 1.38; n = 304) groups. Peripheral blood samples were collected after overnight fast from both male and female volunteers (n = 40 per group) who were categorized as young adult control, old control, MCI and AD. Results AD patients showed lower MMSE scores compared with the young adults, old and MCI groups, and MMSE further decreased at follow‐up examination a year later. In the serum of AD patients, high‐density lipoprotein, alkaline phosphatase activity and bilirubin levels were lower, whereas low‐density lipoprotein, total cholesterol and triglycerides levels were higher. MMSE was positively correlated with high‐density lipoprotein, and negatively correlated with other lipid parameters in AD. Conclusions Hypercholesterolemia is a risk factor for AD that might result in neurotoxicity and cognitive impairment. Dysfunction of lipoprotein and heme metabolism might also provide additional targets for AD diagnosis. Geriatr Gerontol Int 2016; ••: ••–••.