SUMMARY Objective. To determine anti-b2 glycoprotein-I (anti-b2GPI ) and anti-prothrombin (anti-ProT ) antibody levels, and the IgG subclass distribution of anti-b2GPI antibodies, in serial samples from patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS) having initial or recurrent thrombotic/neurological ( T/N ) events during the study period. To investigate the correlations between these antibodies and b2GPI antigen, anticardiolipin antibody (aCL), anti-doublestranded (ds) DNA, C3 levels and disease activity. Methods. Fifty serum samples were identified from seven patients with SLE who had had T/N events during the follow-up from a cohort under long-term follow-up. IgG anti-b2GPI, anti-ProT, aCL, IgG subclasses of anti-b2GPI and b2GPI antigen levels were determined by ELISA. Corresponding disease activity [British Isles Lupus Assessment Group (BILAG)], antidsDNA and C3 levels were compared. Results. IgG anti-b2GPI antibody levels were elevated in six of the patients before and after the T/N events with less marked fluctuations than aCL antibody levels. The predominant subclass of anti-b2GPI antibodies was IgG2 before and after the T/N events. IgG anti-ProT antibodies were negative in all cases. There was a significant but weak correlation between anti-b2GPI and aCL antibodies. No correlation was found between disease activity and IgG anti-b2GPI antibody and b2GPI antigen levels. There were fluctuations in b2GPI antigen levels and a trend to increase after T/N events was observed in some patients. Conclusion. Most of the patients with a T/N event during the study period had IgG anti-b2GPI, but not IgG anti-ProT antibodies. Many IgG aCL-negative samples were found to have IgG anti-b2GPI activity during the follow-up period. The predominant subclass of IgG anti-b2GPI was IgG2, which may have importance in the pathogenesis of APS. b2GPI antigen levels were found to be increased in some patients with SLE after T/N events. IgG anti-b2GPI antibodies may be used as an adjunctive marker of future T/N events in patients with SLE and APS with aCL antibodies and lupus anticoagulant.