Because of the inaccuracy inherent in mass chest radiography it would be desirable to have the chest films examined by the most efficient interpreters and to apply the film material that permits the most efficient interpretation. But before this can be achieved a method for an objective evaluation of efficiency must be developed.In the present paper a method is suggested for evaluating the efficiency of one important activity in film interpretation-namely, the determination of whether or not there is an abnormal opacity in a chest X-ray film. A test procedure is described that may be used for collecting the required data in respect of different interpreters and different types of film material. A discussion of the test data is presented and a method of analysis suggested. It is hoped that the general considerations concerning diagnostic efficiency given in this paper may be useful in the development of methods for evaluating other diagnostic procedures.