Antiphospholipid antibodies (APA) have been known for decades. Their relation to clinical manifestations, primarily thromboses and thrombocytopenia, was recognised in the 1980s. In this clinical study two cohorts of patients, a population-based (84 patients with systemic lupus erythematosus (SLE)) and a hospital-based (87 patients with SLE and 53 with other connective tissue diseases) were investigated for APA and associated clinical manifestations. Anticardiolipin antibodies (ACA) of IgG and IgM classes were found in 13 and 38% of the population-based patients and in 29 and 58% of the hospital-based patients, respectively. The corresponding figures for antibodies against beta2-glycoprotein I (anti-beta2GPI) were 15 and 10% in the population-based patients and 14 and 8% in the hospital-based cohort. Anti-beta2GPI antibodies were always found in association with the corresponding immunoglobulin class of ACA. In both cohorts anti-beta2GPI of the IgG class were associated with arterial/venous occlusion, a result concordant with other studies. A novel finding in both cohorts, however, was an association between thrombocytopenia and IgM anti-beta2GPI.