Raised levels of serum antiphospholipid antibodies have most commonly been reported in patients with systemic lupus erythematosus (SLE). There remains, however, a group of patients with raised antiphospholipid antibody levels who do not have any other well defined disease, but do have clinical features associated with these raised antibodies. The clinical, haematological, and serological features of 20 such patients are reported. Antiphospholipid antibody levels were measured by a solid phase assay for anticardiolipin activity. Fourteen patients had raised IgG antiphospholipid antibodies, 12 had raised IgM, and six had both. Nine out of 19 had raised antinuclear antibody levels; however, non fulfilled criteria for the diagnosis of SLE. Seven patients had a history of venous thrombosis and five of definite or presumed arterial thrombosis-for example, stroke. Of the 15 female patients who underwent pregnancy, 12 experienced fetal loss with up to eight abortions each (mean 3.6). Six individuals had thrombocytopenia and four others had migraines. Other clinical features included livedo reticularis, cardiac and neuropsychiatric disorders, arthralgias, and Raynaud's phenomenon. These findings confirm that the clinical features of individuals with what may be called the 'primary antiphospholipid syndrome' are similar to those in patients with other diagnoses who have raised antiphospholipid antibodies. They indicate that the antiphospholipid syndrome may be related to SLE and other autoimmune diseases, but that, although it frequently overlaps with these disorders, it also exists as a distinct entity.