A 16-year-old girl was hospitalized apparently with Sydenham's chorea. Laboratory investigations failed to reveal any sign of preceding streptococcal infection. However, lupus anticoagulant was demonstrated in plasma. Complete remission followed combined therapy with corticosteroid and anticoagulant drugs. Since the treatment of chorea in systemic lupus erythematosus associated with lupus anticoagulant is completely different from that of Sydenham's chorea, the case illustrates the importance of a thorough laboratory investigation.