A decrease in the number of neuronal profiles in the isocortex of man may be observed on microscopic sections in aging and in degenerative diseases such as Alzheimer's disease. It can be the consequence of a loss of neurons per unit volume or of a reduction of the neuronal volume (i.e. pseudo-loss). This latter possibility has been tested by simulating neuronal atrophy, with sections of various thicknesses. An unfolding algorithm was used for the simulation. The data published in the current literature concerning Alzheimer's disease were treated with the unfolding algorithm. The neocortical pseudo-loss did not exceed a few percentage points, probably much less than the measurement error. New methods of cell counting have been recently proposed to discriminate real from pseudo-loss. They should be used when the risk of dealing with pseudo-loss is high. A chart to assess the percentage of pseudo-loss as a function of perikaryal atrophy is proposed: it relies on the evaluation of the size of the cell relative to the section thickness (relative caliper diameter). This chart may be used to correct cell counts of homogeneous cell populations.