Patients with catastrophic antiphospholipid syndrome (APS) have in common: a) clinical evidence of multiple organ involvement developing over a very short period of time; b) histopathological evidence of multiple small vessel occlusions, and c) laboratory confirmation of the presence of antiphospholipid antibodies (aPL), usually in high titre. Although patients with catastrophic APS represent less than 1% of all patients with APS, they are usually in a life-threatening situation. The rarity of this syndrome makes it extraordinarily difficult to study in any systematic way. In order to correlate all the published case reports as well as newly diagnosed cases from all over the world, an international registry of patients with catastrophic APS (CAPS Registry) was created in 2000 by the European Forum on aPL. Currently, it documents the clinical, laboratory and therapeutic data of more than 400 patients and can be consulted through Internet at www.med.ub.es/MIMMUN/FORUM/CAPS.HTM. The analysis of this registry has allowed the characterization of the clinical and laboratory features of the catastrophic APS as well as the establishment of preliminary criteria for its classification and guidelines for its management.