The loss of well-regulated endothelial cell functioning is followed by adverse changes in a variety of physiological systems, such as the expression of adhesion molecules, maintenance of adequate blood vessel tone and haemostasis. Therefore, a full understanding of endothelial cell biology is essential if the losses of normal function of these systems are to be avoided. The viewpoint presented in this paper suggests that a spectrum between endothelial cell health and disease can be drawn: midway between these two extremes is immunological activation (by, for example, cytokines), which is reversible. Endothelial cell damage or injury (which may be the result of chronic inflammatory activation, hypercholesterolaemia, and/or smoking) are invariably associated with clinical conditions such as hypertension and oedema (and, ultimately, thrombosis and infarction), and are more difficult to reverse. A better understanding of the events, including apoptosis, that lead to vascular dysfunction may be useful in developing our understanding of vascular biology.