Recent evidence suggests that patients with Alzheimer's disease (AD) and vascular comorbidities (VC) perform worse across measures of verbal reasoning and abstraction when compared to patients with AD alone. We performed a qualitative error analysis of Wechsler Adult Intelligence Scale-III Similarities zero-point responses in 45 AD patients with varying numbers of VC, including diabetes, hypertension, and hypercholesterolemia. Errors were scored in set if the answer was vaguely related to how the word pair was alike (e.g., dog-lion: "they can be trained") and out of set if the response was unrelated ("a lion can eat a dog"). AD patients with 2-3 VC did not differ on Similarities total score or qualitative errors from AD patients with 0-1 VC. When analyzing the group as a whole, we found that increasing numbers of VC were significantly associated with increasing out of set errors and decreasing in set errors in AD. Of the vascular diseases investigated, it was only the severity of diastolic blood pressure that significantly correlated with out of set responses. Understanding the contribution of VC to patterns of impairment in AD may provide support for directed patient and caregiver education concerning the presentation of a more severe pattern of cognitive impairment in affected individuals.