There are few detailed descriptions of the coronary arterial patterns in the mouse. Some recent reports on coronary anomalies in mutant mouse models have uncovered the importance of several genes (i.e. iv and connexin43) in coronary morphogenesis. These mutations spontaneously appeared (iv) or were generated (connexin43) in a C57BL/6 background, which is widely used for the development of mutant mice. We have studied the origin and course of the main coronary arteries of two C57BL/6 mouse strains. Unusual anatomical coronary arterial patterns were found, including: solitary ostium in aorta, accessory ostium, high take-off, aortic intramural course, slit-like ostium, sinus-like ostium and origin of a septal artery from the left coronary artery. In humans, some of these conditions are clinically relevant. Most of these patterns, which differ from those observed in wild mice and Swiss albino mice, coincide with those previously found in iv/iv and connexin43 knockout mice. The results indicate that there is variability in the coronary arterial arrangement of the laboratory mouse. Care should be taken when analysing coronary phenotypes of mutant mouse models.