Abstract: A catheter for opening constricted regions in the cardiovascular system includes an expandable dilatation zone comprising three dilatation elements arranged around a support tube. The dilatation elements are of tubular configuration, and each communicates at one end with a passage in the support tube leading to a pressure-suction pump. Constricted regions in the cardiovascular system may be enlarged or opened by introducing the dilatation zone into the affected site in a folded condition and expanding it peripherally therein. Upon expansion of the dilatation elements flow passages are formed between these elements, through which body fluid (for example blood) can pass with little resistance. The supply of body fluid to organs and tissues downstream of the affected site is thereby maintained. Interruption of the dilatation treatment is not required to insure this supply, thus permitting a substantial reduction in the overall duration of the medical procedure.