Four cases of left-sided pleural hydrothorax in connection with severe pre-eclampsia are reported. In addition an increased formation of ascitic fluid was observed in the patients. A long-lasting bed rest in the left lateral recumbent position preceded the discovery of the roentgenologically verified pleural changes. Hypoproteinemia caused by the excretion of large amounts of protein in the urine seems to be a partial etiological factor aggravated by the renal retention of sodium and water. Pleural transudate disappeared spontaneously within two weeks from the delivery. In such cases, immediate diagnostic procedures, e.g. the exclusion of tuberculosis, are unnecessary. The authors think that the phenomenon is common in similar cases but usually remains undiagnosed.