In 1960, Dr. Bayard Clarkson described a woman experiencing sporadic, recurrent episodes of shock and anasarca. Plasma from an acute attack induced a “shock”-like syndrome when injected into rats. The enigmatic “Systemic Capillary Leak Syndrome” (SCLS) named for Dr. Clarkson is characterized by transient and severe, but reversible, hemoconcentration and hypoalbuminemia due to leakage of fluids and macromolecules into tissues. Although < 500 cases of SCLS have been reported in the literature since 1960, the condition is probably under-diagnosed due to lack of awareness and a high mortality without treatment. Allergists should be vigilant of this diagnosis since its presentation can resemble more common plasma leakage syndromes including angioedema or systemic anaphylaxis. Although the precise molecular etiology of SCLS remains unknown, substantial advances over the last five years have increased our understanding of SCLS pathogenesis.