Thirty-seven patients with a presumptive diagnosis of dementia of the Alzheimer type were divided into presenile and senile groups according to the age at which they first received a clinical diagnosis. Although there were no differences in mental status, dementia rating, or chronicity of disease, multivariate analyses of WAIS subtests revealed the presenile subjects to be relatively impaired on Performance subtests. Univariate tests of Verbal, Performance, and Full Scale IQ measures were significantly lower in the presenile group. There was no group effect detected for Digits forward, adjusted for age differences based upon performance of 40 age-matched controls, while the presenile group performed significantly more poorly on backward span. Further, significant differences were detected for an embedded figure task, as well as graphomotor speed. These data suggest that patients who develop a degenerative dementia during the presenile period are more impaired than their senile counterparts on age-adjusted measures of sustained concentration and mental tracking.