Ten anatomical preparations and 15 MRI scans (Magnetic Resonance Imaging) performed on healthy subjects were used to define accurately the lateral attachments and anatomical boundaries of the supraspinatus m. Using 5 frozen specimens sectioned in the plane corresponding to the sagittal oblique MRI plane, it was possible to calculate quantitatively the ratio between the bony contours (O) and muscles (M) of the supraspinous fossa. This ratio was maximal (O/M = 2.4) for the section passing through the plane which included the coracoid process anteriorly and the spine of the scapula posteriorly ("Y" section). Five dissections on unembalmed subjects demonstrated that the postero-lateral origin of supraspinatus m. extended further laterally than classically described. This observation was confirmed in the 15 MRI subjects which showed that the supraspinatus m. may arise as far laterally as the "Y" section on MRI in 53% of cases. A quantitative evaluation of atrophy of the supraspinatus m. using MRI is possible with a knowledge of these two parameters.