The increasing application of routine examinations of all kinds is based on the fact that many serious illnesses do not produce clinical symptoms until late in their course. Any simple screening procedure increasing our case finding of malignant lesions is therefore valuable. Plain roentgenograms of the abdomen will give information about the presence of a mass lesion of the kidneys much more frequently than is generally appreciated. The observation that a renal mass was often an incidental finding in a department where many routine gallbladder and gastrointestinal studies were done drew attention to the value of a scout film of the abdomen for study of renal contours. For the full diagnostic harvest of a scout film, however, two conditions must be fulfilled: (a) The film must be properly exposed for upper abdominal density and (b) the radiologist must have formed the habit of carefully scrutinizing the renal contours in their entirety (including the medial aspect) and be aware of deviations from the normal. The renal outline will be recognizable on a properly executed film in a high percentage of cases. Among 122 abdominal films obtained in a random group of 102 consecutive patients, the right renal outline was satisfactorily visualized in 92 (75 per cent) and the left renal outline in 79 (65 per cent). Among the 102 patients the kidney was seen in at least one of the examinatons, on the right in 80 (78 per cent) and on the left in 71 (69 per cent). Reasons for non- or poor visualization of renal contours are obliteration by fecal matter in the colon, absence of perirenal fat in very thin individuals, and obliteration of renal contours by overlying masses. If the renal contours are not visible on the available scout film, better preparation, some variation of exposure factors, or oblique films may result in their demonstration in some instances. There still remain cases, however, in which satisfactory results will not be obtained in spite of repeated attempts, unless tomography is employed. The analysis of proved renal mass lesions to be presented here was undertaken with the idea that it might provide information as to the value of a scout film of the abdomen for their diagnosis. The series comprises tumors originating in the parenchyma and cysts. Tumors arising in the renal pelvis are not included. Key (17) was the first to diagnose a renal tumor on a plain film of the abdomen in 1910, long before contrast studies had been introduced. Since then many authors (3–6, 8–10, 14, 15, 19, 24, 29, 31) have mentioned that a renal neoplasm may deform renal contours and that cysts are seen occasionally on plain roentgenograms. To our knowledge, however, the only data on the incidence of visibility of renal masses on scout films are those of Johnsson (16), whose main interest was to prove the value of properly performed compression urography as compared to retrograde pyelography for the diagnosis of renal neoplasms.