PURPOSE: To compare image quality of storage phosphor digital radiography(DR) with film-screen radiography in portable chest imaging, and to assess the minimum X-ray dose that can be applied to DR in adults without image degradation, and also to compare image qualities of low dose and standard dose DR. Materials and Methalos: A geometrical phantom similar to the human thorax was imaged by a portable radiographic unit with fixed kVp and variable m^s in both film-screen and DR systems. Three radiologists scored the images by four grades in four categories of 1) contrast between mediastinum and lung, 2) definition of the nodule in the lung, 3) definition of another nodule through the mediastinal shadow, and 4) grainess(noise : assessed only in DR). Additionally, portable chest images were obtained in 10 patients in a intensive care unit by film-screen, standard dose and half dose DR in consecutive days. The same readers scored the images by four grades in six categories of 1) the lungs and hila, 2) the mediastinum, 3) subphrenic area. 4) musculoskeletal shadow, 5) tubes and lines, and 6) grainess (only in DR). The images with superior quality were assessed by paired t-test. RESULTS: In phantom study, the minimum dose of digital images scored 3 or more by all readers was 39% of the standard dose. In patient study, DR was superior to film-screen radiography in all categories except tube and line. Low dose DR was not inferior to standard dose DR in five categories other than grainess to two readers or more. CONCLUSION: In portable chest imaging, storage phosphor DR image was superior to conventional film-screen radiography and half dose DR was comparable to standard dose DR despite of more noise.