Roentgenographic examinations of 1800 shoulder girdles, made in the sagittal plane, disclosed twenty-one typical and four atypical cases of os acromiale. At least six of these cases have been contested as being possible fractures. An additional study of 1000 cases gave a frequency of 2.7 per cent., 62 per cent. being bilateral. A supero-inferior examination facilitates the making of a differential diagnosis.