Our purpose in this study was to compare the energy absorbed in a computed radiography (CR) plate with that absorbed by a film-screen system over the diagnostic X-ray tube voltage range. A Fuji ST-II and Fuji HGM/UR2 were selected as a CR plate and film-screen system, respectively. The X-ray energy absorbed by the phosphor layer (per mA per unit area) was calculated theoretically as an index of the radiographic sensitivity by use of the incident X-ray photon spectrum with the Birch-Marshall formula and the sensitivity spectrum for the range of 40-140 kV. The radiosensitive media were treated as layers involving mass loading. The relative radiographic sensitivity of the ST-II CR plate best approached that of the HGM/UR2 film-screen at 60 kV; it was considerably lower for the ST-II CR plate on both sides of 60 kV. The relative sensitivities at 40 and 140 kV of the ST-II CR plate to the HGM/UR2 film-screen decreased by 16 and 30%, respectively. This result implies that, in the CR system, the mAs values must be increased by 16 and 30% at 40 and 140 kV, respectively. These results were explained by the relative positions of the K-absorption edges of the phosphors. The theoretically calculated result was in good agreement with the experimental result obtained with an acrylic resin phantom. These results would be useful in preventing under- or overexposure in a CR system and thus controlling the dose administered to the patient.