The systemic capillary leak syndrome (SCLS) is a rare disease of reversible plasma extravasation and vascular collapse accompanied by hemoconcentration and hypoalbuminemia. Its cause is unknown, although it is believed to be a manifestation of transient endothelial dysfunction due to endothelial contraction, apoptosis, injury, or a combination of these. Fewer than 150 cases of SCLS have been reported, but the condition is probably underrecognized because of its nonspecific symptoms and signs and high mortality rate. Patients experience shock and massive edema, often after a nonspecific prodrome of weakness, fatigue, and myalgias, and are at risk for ischemia-induced organ failure, rhabdomyolysis and muscle compartment syndromes, and venous thromboembolism. Shock and edema reverse almost as quickly as they begin, at which time patients are at risk for death from flash pulmonary edema during rapid fluid remobilization. Diagnosis is made clinically and by exclusion of other diseases that cause similar symptoms and signs, most notably sepsis, anaphylaxis, and angioedema. Acute episodes are treated with vasopressor therapy and judicious fluid replacement, possibly with colloid solutions for their osmotic effects, to prevent the sequelae of underperfusion. Between episodes, patients may be treated with theophylline and terbutaline, which clinical experience suggests may reduce the severity and frequency of acute episodes. Prognosis is uncertain, but patients who survive an initial severe SCLS episode are estimated to have a 10-year survival rate greater than 70%. Much remains to be learned about SCLS, and clinicians should consider the diagnosis in patients with unexplained edema, increased hematocrit, and hypotension.