We investigated the number of tyrosine hydroxylase (TH)-immunoreactive neurons in the C1 and A2 regions of the medulla, the sites of the baroreflex arc, in 7 patients with multiple system atrophy (MSA), 8 with Parkinson's disease (PD), 9 with amyotrophic lateral sclerosis (ALS), and 12 age-matched normal subjects to analyze the relationship between cardiovascular dysfunction and medullary catecholaminergic neurons. Orthostatic hypotension (OH) was marked in all the MSA patients and moderate in three PD patients. Three of the five ALS patients who had been on respirators showed lability of blood pressure; paroxysmal hypertension and nocturnal hypotension without compensatory tachycardia. All the MSA patients showed extremely marked decrease of TH-immunoreactive neurons in both the C1 and A2 regions. In the patients with Parkinson's disease, numerous TH-immunoreactive neurons contained Lewy bodies that were immunostained by antibody to TH. TH-immunoreactive neurons were decreased very markedly in the A2 regions of two patients with OH, and three patients without OH showed fairly marked decreases in the C1 or A2 region. In contrast, the number of TH-immunoreactive neurons in ALS was the same as in normal subjects. In MSA and some PD patients, orthostatic hypotension may partly be due to the involvement of the medullary catecholaminergic neurons. The lability of blood pressure in ALS probably is not related to the medullary catecholaminergic neurons.