A female patient with a history of migraines and chorea developed polyarthralgia at age 24 and was diagnosed with rheumatoid arthritis. In 1991 she was hospitalized because of impaired renal function and hypertension. Examination revealed thrombocytopenia and the presence of lupus anticoagulant. Antinuclear antibody was weakly positive, but anti-DNA antibody was negative, and no decrease in leukocyte count or complement level was observed. Rheumatoid arthritis with antiphospholipid syndrome was diagnosed. Renal biopsy showed renal thrombotic microangiopathy. This renal lesion was considered to be associated with antiphospholipid syndrome. Cyclophosphamide pulse therapy and anticoagulation therapy decreased proteinuria and improved renal function.