The metabolic O-methylation of endogenous catecholamines and other catechols catalyzed by catechol-O-methyltransferase (COMT; EC 2.1.1.6) was first described by Dr. Julix Axelrod and his colleagues almost half a century ago. In the past several years, research interest in this catechol-metabolizing system has been renewed because of its potential pathophysiological and pathogenic significance in estrogen-induced hormonal cancers, in the development of degenerative brain disorders, as well as in the development of cardiovascular diseases. In this review paper, I provide a brief overview of the COMT metabolic system, with particular attentions being paid to the following three areas: (i) the regulation of this catechol-metabolizing system by endogenous regulatory factors (mainly S-adenosyl-L-homocysteine and homocysteine) as well as by exogenous factors such as dietary phytochemicals; (ii) decreased metabolic O-methylation of endogenous catecholamines as an important risk factor for the development of neurodegenerative disorders such as Parkinson's and Alzheimer's diseases in the elderly and also as a risk factor for the development of a variety of cardiovascular diseases; and (iii) the relative importance of the COMT-catalyzed O-methylation metabolism of endogenous catechol estrogens in the causation and prevention of estrogen-induced hormonal cancers. Some unifying hypotheses are also discussed in this paper with the hope that they may provide useful mechanistic insights into our understanding of the biological functions that are associated with this important metabolic system.