Antiphospholipid syndrome (APS) is an entity characterized by recurrent thrombotic events and may occur spontaneously or in the context of systemic lupus erythematosus (SLE). We describe an English Canadian family in whom the propositus, a woman with Graves' disease and SLE, was found to have a lupus anticoagulant and anticardiolipin antibody (aCL). A brother with deep vein thrombosis, pulmonary emboli, bilateral adrenal hemorrhage and thrombocytopenia, circulating anticoagulant and aCL had a positive antinuclear antibody and Coombs' test, but no other features of SLE. Fourteen members of 3 generations of this family underwent clinical assessments, serological testing and HLA typing. The propositus' mother had a family history of autoimmune thyroid disease and the father had aCL, but was asymptomatic. The thyroid disease and the SLE were associated with HLA-B8, DR3 haplotype. The aCL and the anticoagulant were associated with HLA-B60, DR4 haplotype. Both these haplotypes were present in the propositus. Among the other 4 carriers of the haplotype B60, DR4, 3 demonstrated significant titers of aCL. Our findings support the reported association between APS and the HLA haplotype DR4 in patients of English descent with SLE.