To develop a tool for precisely assessing dementia severity, items should be selected according to their relationship to the overall progression of the disease. Using an item characteristic curve analysis (ICC), items were examined from the Folstein Mini-Mental State Exam (MMSE), a useful clinical tool for evaluating dementia. MMSE data were available for 86 patients who met DSM-III criteria for primary degenerative dementia -- possible or probable Alzheimer's disease (AD). A logistic regression analysis of the probability of correct performance on an item, given the total MMSE score, yielded statistics for difficulty and discrimination. These statistics were interpreted respectively as indicators of the point in the progression of the illness at which the mental function tested by that item is lost and the rapidity of that loss. The data indicated a systematic progression of the development of symptoms in AD related to decline of memory function. Temporal orientation was lost before spatial and object orientation, and recollection of words was lost before ability to repeat them. ICC can help to delineate the loss of mental functions during the course of AD.