The effect of white matter lesions in magnetic resonance imaging or vascular atherosclerosis on cognitive function is not fully understood in Alzheimer disease (AD). In this investigation, we examined the influence of white matter lesions on cognitive decline in AD. A total of 142 patients with AD (44 men, mean age 65.7 + 7.6 years; mean education period 7.8 + 5.0 years) were included. Patients were divided into 4 groups based on the severities of white matter hyperintensities (WMH) in brain magnetic resonance images (MRI) using Fazekas scale. Cognitive functions were determined using the Korean version of the Mini-Mental State Examination (K-MMSE) and the Clinical Dementia Rating (CDR) scale before acetylcholinesterase inhibitors were administered. Of the 142 patients, 30% (43/142) had no white matter signal abnormality (grade 0). Fourteen percentage (20/142) were grade 1, 42% (59/142) grade 2, and 14% (20/142) were grade 3. Mean K-MMSE scores declined as MRI grades increased to grade 2 and 3 compared to grade 0 (P < .01). Clinical Dementia Ratings were also aggravated by MRI grade. These results remained significant after adjusting for compounding factors affecting cognitive functions; sex, age, number of years in full-time education, hypertension, diabetes, hypercholesterolemia, smoking, and atrial fibrillation. The presence of WMHs were associated with score of MMSE and CDR impairment in patients with AD. These features could be a correctable factor hastening cognitive decline in AD.