Computed tomography (CT) is superior for the detection of substances with low radiolucency in comparison with abdominal roentgenograms. In the present study, medical chart review was retrospectively performed for patients who were admitted and underwent plain CT including the stomach on arrival to investigate whether CT is useful for diagnosing overdose (OD). The subjects were divided into patients with OD who did not undergo gastric lavage (OD group) and those without OD (Control group). The presence of a radiopaque area (Hounsfield number over 100 on a range of interest of 3 mm2) in the stomach on CT was defined as a positive finding. The average Glasgow Coma Scale in the OD group (n=11) was significantly lower than that in the Control group (n=137). Positive findings on CT were found more frequently in the OD group than in the Control group (100 vs. 19.7%, p＜0.0001). Based on the finding of a high-density deposition in the bottom of the stomach, the CT predicted OD with 98.5% specificity. Accordingly, CT findings of a high-density deposition in the stomach of a patient with a diminished consciousness may suggest the presence of a recent overdose.