The present study employed multidimensional scaling and ADDTREE clustering analyses to derive the cognitive maps and clustering representations of normal elderly controls (NC), patients with Alzheimer's disease (AD), and patients with Hun-tington's disease (HD); the analyses were performed on subjects' responses in a category fluency task that involved generating animal names for 60 sec. A measure of the proximity of animal names was used as an index of associational strength; MDS and ADDTREE estimates were based on this measure. A comparison of the NC, AD, and HD subjects' cognitive maps suggests that the semantic network of AD patients is abnormal in two ways. First, the organization of the semantic network is disrupted. Second, new abnormal associations and clusterings are formed. These results support the notion that AD is characterized by a breakdown in the structure of semantic knowledge and not primarily by a deficiency in the accessibility of semantic information.