Diagnosis of pleural effusion (PE) on supine chest X-ray (SCXR) is considered difficult. This study aimed at evaluating the accuracy of the diagnosis of PE on SCXR and was performed in two phases. At phase 1, a formula for the sonographic estimation of the volume of PE was established by correlating 24 measurements (in 7 patients in whom complete drainage was achieved) with the drained volumes. At phase two, 112 consecutive SCXRs were supplemented by sonography of the chest. The films were evaluated for the presence of PE and for the presence of different radiologic signs of PE. Sonography showed PE in 41 right and 30 left hemithoraces. The overall accuracy of the diagnosis of PE on SCXR was 82 %. Only one of the undiagnosed PEs had a volume of > 300 ml. The most accurate signs were increased density of the hemithorax, blunted costophrenic angle, and loss of the hemidiaphragm silhouette.