According to definition, collateral circulation is an alternative to major vascular flow which has become dysfunctional. Collateral channels, initially unused, are being formed due to impossibility of the main blood vessel to provide normal coronary flow. Recent controversies about collateral circulation are mainly based on their functional significance. The aim of this work was to evaluate the function of collateral circulation in patients after the first postero-inferior myocardial infarction i.e. whether adequate collateral circulation may reduce the size of myocardial infarction and prevent development of ischemic mitral regurgitation.The investigation included 128 patients (pts) treated at the Institute of Cardiovascular Diseases in Sremska Kamenica during 1997 and 1998. The investigation group (I) included 64 pts, 58 males and 6 females, mean age 54.42 years. The control group (C) included 64 pts, 56 males and 8 females mean age 51.71 years. In all patients the first posterior, inferior and postero-inferior myocardial infarction were proven during 1-year period. Cineventriculography confirmed kinetic disturbance of the area with or without mitral insufficiency. Degree of mitral regurgitation was evaluated according to Seller's criterion. Alterations on the right coronary or circumflex branch of the left coronary artery were confirmed, but significant stenotic alterations were not verified on the anterior descendent branch of the left coronary artery. Alterations on epicardiac coronary vessels were presented as total coronary score--modification according to Benc (18 segments) while numerical values for stenosis according to Kaltenbach. This value represents the total coronary score (SCORE-A) including alterations before and after occlusion. Gensiny's principle modified for multiplication factor according to Benc and numerical value according to Kaltenbach were used for evaluation of collateral circulation. Levin's classification was used for evaluation of collateral circulation quality. We used quantitative classification according to Cohen.Distribution of mitral regurgitation was not statistically and significantly frequent in the subgroup of patients with lesions on the right coronary artery in relation to subgroup of patients with combined lesions on the right coronary artery and circumflex branch of the left coronary artery (p > 0.05). None of the investigated patients with extensive lesions (lesions on ACD and RCX) had mitral regurgitation of IV degree, so the number of investigated patients was not adequate for statistical evaluation. We did not find a statistically significant difference in the percentage of collateral circulation between the investigation (59.4%) and control group (62.5%) (p > 0.05). Poorly developed collaterals were statistically and significantly more frequent in the investigation group (60.5%) in relation to control group (10%) (p < 0.01). The percentage of moderately developed collateral circulation was similar in both groups. Well developed collateral circulation was statistically and significantly more frequent in control group (52.5%) in relation to investigation group (7.9%) (p < 0.01). The extension of stenotic alterations expressed as total coronary score A was statistically and significantly higher in subgroups of investigated patients with combined alterations on ACD and RCx, proving the sensitivity of our score system in smaller extensity of stenotic lesions on epicardial coronary blood vessels. Collateral circulation was significantly better in patients with ACD occlusion (100%) in relation to patients with significant stenosis or ACD subocclusion (33.3%) (p < 0.01).The functional role of collaterals has not been explained yet. The efficacy of coronary collateral vessels and mechanism of adequate compensation of regional perfusion in a position distal from the occluded vessel is highly controversial. (ABSTRACT TRUNCATED)