The prevalence of a number of clinical features occurring in patients with early- (before age 65) and late-onset primary degenerative dementia of the Alzheimer type (PDDAT) are compared. The data from a total of 65 patients demonstrated a greater prevalence of language disturbance, a disproportionate number of left-handers, and a much shorter relative survival time in the early-onset group. Taken together with other data, these findings indicate a degree of heterogeneity in the clinical features of patients with PDDAT and suggest a possible heightened selective vulnerability of the left hemisphere in early-onset cases. We interpret the data as raising the possibility that the current view of PDDAT as a unitary disorder may not be entirely valid.