The purpose of this prospectively performed study was the angiographic visualization of the posterior right diagonal artery (PRDA) and its differentiation from the epicardial branches of the right coronary artery (RCA), that is, the right marginal artery and the posterior descending artery (PDA). The authors prospectively studied the angiographic findings of 607 patients who underwent coronary angiography. The incidence of the angiographically demonstrated PRDA and its distinction from other epicardial branches arising from the distal third of the RCA was the main point of interest. Two types of PDA in those cases where PRDA was present were also demonstrated. Of the patients examined, 535 had dominant right coronary circulation, 59 had left dominant coronary circulation, and 13 had balanced coronary circulation. PRDA was present in 81 patients with right dominant coronary circulation (15.1%), in 2 patients with balanced coronary circulation (15.4%), and in none with left dominant coronary circulation. PRDA was revealed in 48 (40%) of 120 patients with a short PDA and in only 33 (8%) of 415 patients having long PDA. It is imperative to search always for the PRDA, when one is studying coronary arteriographies, bearing in mind that this artery may perfuse the inferior part of the posterior interventricular septum and the adjoining are, depending on the type of PDA.