The clincical and laboratory features of a sixth patient with periodic systemic capillary leak syndrome are reported. During an attack metabolic studies demonstrated a marked shift of plasma (10 to 70%) from the intravascular to the extravascular space resulting in hemoconcentration (highest hematocrit of 82). At the termination of the attack there was a return of the electrolytes, water and proteins to the intravascular compartment. The cardiovascular, renal and endocrine compensation was appropriate to this insult and no underlying abnormalities were demonstrated in these systems. The effector pathways of coagulation, complement, bradykinin generation, prostaglandins and histamine metabolism did not appear to be responsible for the altered capillary permeability. The patient was not missing inhibitors of these same pathways. The only persistently abnormal finding was a monoclonal IgG gammopathy. However, further studies of this paraprotein did not uncover a link between it and the abnormal capillary permeability. Five similar cases are reviewed; at least four and possibly all of these patients also had an IgG paraprotein. Treatment of these attacks was unsuccessful. Attemps to prevent the episodes with a wide variety of therapeutic agents failed. Treatment of the acute attacks with administration of intravenous fluids, did not maintain an adequate intravascular volume and may lead to fluid overload upon return of normal capillary integrity. Pressor agents were of no apparent value and may cause increased cardiac irritability. Although the clinical features and pathophysiology of the capillary leak syndrome have been defined, the etiology remains unknown.