Objective. Left circumflex (LCx) artery originating from the right coronary arterial (RCA) system has been reported as the most common form of anomalous origination of a coronary artery from the opposite sinus (ACAOS). However, some studies claim that RCA originating from the left coronary sinus (LCS) is the most frequent form. The aim of this study was to determine the most common type of ACAOS in a single center. Materials and Methods. The database of the catheterization laboratory was retrospectively searched. All patients who were performed coronary angiography between 1999 and 2006 were included to registry. All examinations were carefully analyzed to determine the most frequent type of ACAOS. Results. We detected ACAOS in 35 cases (16 RCA originating from the LCS, 13 LCx from the RCS or the RCA, and 6 others) out of 5165 coronary angiograms. The most common form was RCA originating from LCS. Moreover, we revealed that 5 cases with RCA originating from the LCS were previously misdiagnosed and not reported as a coronary anomaly. Conclusions. RCA originating from the LCS was the most common form of ACAOS in our registry. The high change of misdiagnosis or underreporting of this anomaly could have biased the true prevalence.