The age-related cell loss of the nucleus basalis of Meynert is of considerable importance because loss of its neurons may be followed by cognitive decline. Compared to the number found at ages 16-29 years, we found that 50% of the total population of neurons is lost by 90 years of age. This change in number is accompanied by modifications in the morphometric features, including a 17.3% increase in cell size by 60 years of age as compared with values at 16 years, and followed by a gradual decline. Topographic differences were seen both in the neuronal loss and in morphometry: in relation to the youngest group, the posterior subdivision is the most severely affected by 90 years (64.5% decrease in number and 10% reduction in neuronal size), followed by the intermediate subdivision (42% loss of neurons accompanied by 4% increase in cell size). In the anterior subdivision no significant decrease in the number of neurons could be detected, although a 15% increase in cell size occurred.