Neurotransmitter-related markers were examined in Alzheimer-type dementia (ATD) and studied whether or not there is biochemical difference between the early- and late-onset sub-groups. Postmortem brains were obtained from neuropathologically diagnosed ATD patients and control subjects with no clinico-neuropathological findings indicative of neuropsychiatric diseases. Neurochemical data in the early- and late-onset groups were compared to the age-matched younger and older control groups, respectively, and expressed as a percentage of the mean value in the respective controls. Choline acetyltransferase activity and concentrations of serotonin and noradrenaline were more severely depleted in the early-onset ATD group than in the late-onset ATD group. These findings indicative of heterogeneity of ATD patients were discussed from the pathogenetic and therapeutic viewpoints.