Studied the performance of patients with executive dysfunctions on a newly developed cognitive estimation task. 50 patients with Alzheimer's disease (AD, mean age 68 years), 50 patients with Korsakoff's syndrome (KS, mean age 56 years), and 50 healthy controls (mean age 65 years) completed the "Test zum Kognitiven Schaetzen" (TKS), a German cognitive estimation test in which subjects estimate the size, weight, and quantity of objects as well as the duration of specific events. Additionally, a neuropsychological test battery assessed intelligence, general knowledge, attention, memory, executive functions, visuo-constructive ability, language comprehension, and general cognitive state. The estimation performance and distribution of various error types were compared across groups. Both patient groups showed impaired performance on all estimation dimensions and a greater number of "bizarre" errors than controls, with AD patients performing generally worse than KS patients. General knowledge correlated with estimation performance, and the amount of bizarre and unit errors correlated with general knowledge, working memory, and executive functions. The results are interpreted in terms of a model of cognitive estimation involving the semantic memory system as well as executive functions.