Ten percent of 667 consecutive systemic lupus erythematosus (SLE) patients were considered to have definite antiphospholipid syndrome (aPLS) because they had two or more antiphospholipid (aPL)-related clinical manifestations and aPL titers more than 5 SD above the mean of normal controls. Another 14% had either one aPL-related manifestation but high titers of the antibody or two manifestations and low aPL titers (probable aPLS). One fourth of the patients had no manifestations but high titers, one manifestation and low titers, or two or more manifestations and negative aPL titers ("doubtful" aPLS); the other half were considered negative for aPLS. In patients with high-titer aPL, the number of aPL-related manifestations was influenced by disease duration and number of pregnancies, indicating potential mobility of category with time or with risk of recurrent pregnancy loss. Patients with two or more manifestations but variable aPL levels differed in immunosuppressive treatment and in the number of times they had been tested, indicating potential mobility of category with lower treatment and/or further aPL testing. Patients with definite aPLS had increased risk of cutaneous vasculitis, peripheral neuropathy, seizures, psychosis, transient ischemic attacks, and leukopenia. In 11 of 52 SLE patients with definite aPLS the initial manifestation was related to aPL, and in 16 it concurred with an unrelated one. Only two patients fulfilled criteria for aPLS before having other evidence of SLE. The authors conclude that aPLS occurring within SLE is part of the disease rather than an associated condition and propose the use of definite and probable classification categories. These criteria, with appropriate follow-up and clinical and serological exclusion clauses for potential primary conditions, could also be applied to primary aPLS.