We studied the pigmented neurons of the substantia nigra (SN) from 8 controls and 20 patients with Parkinson's disease (PD) using a computerized morphometric methodology. On the basis of neuronal topography, several anatomic regions were outlined in the SN. In these subregions the area, perimeter, diameter of the cell bodies and cell numbers were measured and were counted in the controls and PD patients. The measurements were made at the level of the exit of the third cranial nerve from the brain stem. In PD patients, when the whole SN was considered, the mean area, mean perimeter and diameter of the pigmented cell bodies were significantly reduced by 35%, 20% and 21% respectively from the control mean values. Regionally, the pigmented neuron area in the medial ventral part (VM), medial dorsal part (DM), lateral ventral part (VL), lateral dorsal part (DL) and pars lateralis part (PL) showed a significant reduction of 33-41% as compared to controls. In these subregions, a significant decrease in PD patients from the control mean values was seen both in the pigmented neuron perimeter, by 19-26%, and the diameter by 19-25%. This decrease in cell size suggests that, in PD patients, the remaining pigmented neurons in the SN are in a process of degeneration and atrophy. In PD patients the number of pigmented neurons in the whole SN decreased about 76% from control values. Evaluation of the influence of cell size on the apparent quantity of cells in sections indicates, however, that in PD patients the impact of true loss of pigmented neurons is far more dramatic than the impact of their decrease in size.