The majority of anatomical structures within the heart during typical atrial flutters' ablation, right sided accessory pathway ablation or slow pathway ablation are invisible or blurred. Therefore it is very important to know in details interior right atrial structures during such procedures. In the neighborhood of coronary sinus orifice small concavity is visible. This area, called subthebesian fossa, is placed between the os of coronary sinus, the orifice of vena cava inferior and tricuspid annulus. The fossa is on the way of typical atrial flutters' reentrant circuit and is placed next to the isthmus area, which has become a target site for ablative therapy. Regarding the facts mentioned above we decided to examine the topography of this concavity in relation to neighboring structures. Research was conducted on material consisting of 45 human hearts of both sexes, from 19 to 71 years of age. The hearts came from patients whose death was not cardiologic in origin. The topography of the fossa was examined in relation to coronary sinus orifice (diameter A), vena cava inferior orifice (diameter B) and the attachment of the posterior leaflet of the tricuspid valve (diameter C). Besides we measured two perpendicular sizes in the inlet plane of the fossa. There were the longest size (diameter D) and the shortest size of the fossa (diameter E). We also defined deepness of the fossa (diameter F). Diameter A was from to 2 to 7mm (avg. 4.9 +/- 1.4 mm), diameter +/- from 2 to 8mm (avg. 4.0 +/- 1.6 mm) and diameter C from 5 to 9 mm (avg. 7.0 +/- 1.5 mm). The longest size in inlet plane of the concavity (diameter D) was from 12 to 18 mm (avg. 14.1 +/- 1.7 mm) and shortest size (diameter E) was from 7 to 14 mm (avg. 9.0 +/- 1.7 mm). The deepness of the fossa (diameter F) was from 2 to 7 mm (avg. 4.8 +/- 1.2 mm). The subthebesian concavity is inconstant anatomical structure, occurring in all forty five examined hearts (100%). The shape and sizes of the subthebesian fossa were variable in examined group of hearts. Our data suggest that differences in diameters between subthebesian fossa and neighboring structures may have clinical importance during ablation procedure.