Clinical and electron microscopic studies of two female patients are reported who, in addition to the symptoms and signs of idiopathic edema, had pericardial (and pleural) effusions. In one patient pericardiectomy had to be performed, in the other patient pharmacotherapy with spironolactone was effective. In these patients the protein-concentrations of edema fluid (25 and 12 g/l) and pericardial fluid (55-61 and 48 g/l) were relatively high; the distribution space of labelled protein was increased; blood volumes were low-normal or decreased. Electronmicroscopy of the microvasculature showed identical alterations in both cases. In the cutaneous vessels the endothelium developed numerous abnormal cytoplasmic processes and intercellular "gaps". In the capillaries of skeletal muscle our findings indicate an increase in vesicular transport. We suggest the electron microscopic alterations to be the morphologic correlate for the increased transport and extravascular accumulation of protein. Changes as they are described here in humans with idiopathic edema have been documented before in animals with experimental edema.