Preliminary classification criteria for the antiphospholipid syndrome (APS) were formulated during a post-conference workshop held October 10, 1998 in Sapporo, Japan, following the Eighth International Symposium on Antiphospholipid Antibodies. Members of the workshop panel included all of the authors and the individuals listed in the Appendix. Widespread recognition of the clinical importance of antiphospholipid antibodies (aPL) dates from the seminal work of Harris, Gharavi, and their colleagues in 1983 (1). Clinical and experimental evidence, reviewed at the Sapporo symposium and at the Seventh International Symposium on Antiphospholipid Antibodies held in 1996 (2–11), indicates that aPL are significantly associated with and may have a causative role in vascular thromboses and pregnancy losses. However, the mechanisms by which the associations occur are not well understood and appear to be heterogeneous, reflecting, in part, the heterogeneity of aPL antibodies. The need to develop consensus criteria for APS has been heightened by the diversity of clinical and basic science disciplines that have made major contributions to knowledge of APS; these include rheumatology, immunology, obstetrics, neurology, hematology, radiology, protein and lipid chemistry, and molecular biology. Notwithstanding 4 previous proposals for criteria (12–16), there remains a wide perception among investigators in the field that there is a need for uniformity and consensus in classifying APS. An international multidisciplinary symposium on antiphospholipid antibodies has been held every 2 years since 1984 (17). Recent symposia have each been attended by more than 250 registrants, and at each symposium 250–300 abstracts have been presented. The post-symposium workshop on classification criteria in Sapporo was attended by 56 investigators, including 27 members of the advisory board of the symposium. Eighteen investigators who are acknowledged experts in the field made invited presentations,