Accumulation of Tc-99m labeled phosphonate bone scanning agent in a pleural effusion usually shows a mild and diffuse increase in radioactivity of the involved thorax. A malignant neoplasm was thought to account for this accumulation. The photon deficiency of the hemithorax on Tc-99m HMDP bone scintigraphy was shown in a case of massive pleural effusion, which was proved by autopsy to be due to metastatic breast carcinoma in the pleura. Two factors caused these scintigraphic findings: 1) a large amount of fluid in the pleural cavity caused photon attenuation; 2) the higher body background in the noninvolved hemithorax and other areas of the body was due to renal dysfunction resulting from chronic pyelonephritis. Whether the accumulation of the radiopharmaceutical agent in the pleural effusion was malignant or benign could not readily be distinguished.