An 11-year-old boy developed florid choreic movements in his right extremities after having had an episode of febrile illness. He was evaluated at our hospital where MRI disclosed a honeycomb-like low signal intensity area rimmed by a thin Gd-enhanced layer in the left putamen. Arteriography revealed the lenticulostriate arteries being segmentally narrowed and a "ground glass" staining was observed in the left putamen in late venous phase. Sydenham's chorea, that had been the initial impression, was not substantiated because of negative pharyngeal culture for streptococci, negative ASLO/ASK titers and because of lack of clinical stigmata of rheumatic fever. However, prothrombin time was prolonged, and activated partial thromboplastin time (APTT), that had been also prolonged, was not normalized by adding healthy serum, indicating the presence of lupus anticoagulant. VDRL was false positive and anticardiolipin antibodies, both IgM and IgG classes, were also detected. However, systemic lupus erythematosus was unlikely in view of negative antinuclear antibody and LE phenomenon. He deteriorated rapidly due to development of severe bilateral chorea, thereby he was unable to walk or feed himself. He received a 3-day course of mega-dose intravenous methylprednisolone, that temporarily lessened the chorea, but soon it became worse. A second course of mega-dose methylprednisolone was given, followed by daily maintenance dose of prednisolone. His chorea gradually improved in severity and after 2 months only a trace of choreic movements was detected in his hands. He has been followed at our outpatient clinic where he no longer shows chorea and the APTT has improved to nearly normal time.(ABSTRACT TRUNCATED AT 250 WORDS)