We report the case of a 47-year-old man who was admitted because of syncope. Upon hospital admission, he rapidly developed circulatory shock with generalized edema and a severe hemoconcentration with a hematocrit of 70%. The condition was stabilized with infusion of 17 l of cristalloid fluids over a period of 24 h. After ruling out possible secondary causes, the diagnosis of a systemic capillary leak syndrome--a severe transient endothelial barrier dysfunction of unknown origin--was made. A triad of hypotension, hemoconcentration (hematocrit >60%) and macromolecular extravasation is the typical finding; furthermore, a strong association with monoclonal gammopathy of unknown significance (MGUS) is described.