The present study provides qualitative and quantitative investigations of the norepinephrine (NE) neurons in the locus coeruleus (LC) in two neurodegenerative disorders, the senile dementia of the Alzheimer type (SDAT) and Parkinson's disease (PD). The group of PD subjects was subdivided into cases without dementia (P - D), cases with dementia, L-dopa responsive (P + D), and cases with fulminant dementia whose motor disorder symptoms were L-dopa nonresponsive (P + D/L-dopa non-responsive). NE neurons were demonstrated by immunocytochemistry against tyrosine hydroxylase (TH). Quantitations of neuronal parameters and cell numbers and three-dimensional reconstructions of the LC were carried out with a computer-assisted system. In SDAT cases, the rostrocaudal LC length (13 +/- 2.2 mm) is shorter than in controls (14.9 +/- 1.4 mm). The four basic LC neuron classes found in the normal human brain (large multipolar, large "bipolar," small multipolar, and small "bipolar" neurons; Chan-Palay and Asan: J. Comp. Neurol. this issue) are recognizable, but many cell somata are swollen and misshapen with fore-shortened, thick, and less branched dendrites. LC neuron numbers are reduced (between -3.5% and -87.5%). Neuron loss is greatest in the rostral part, less in the middle, and least in the caudal part. In PD cases, the rostrocaudal length (12.4 +/- 1.5 mm) is shorter than in SDAT and controls. The neuronal morphology is more severely altered than in SDAT. The basic neuron classes are hardly distinguishable. Most cell bodies are swollen; they frequently contain Lewy bodies; and the dendrites are short and thin with absent or reduced arborizations. Neuron numbers are more reduced than in SDAT (between -26.4% and -94.4%). Alterations are as severe caudally as rostrally in P - D, and P + D/L-dopa nonresponsive cases. P + D cases are more severely affected rostrally. The presence of depression in SDAT and Parkinson's patients is accompanied by the greatest loss of LC neurons. On the basis of morphological alterations of the TH-immunoreactive neurons, and the degree and topographical distribution of neuron loss, a differentiation is possible between the LC in normal brain and that in SDAT and PD for diagnostic purposes.