A 2 year-old female developed an acute bleeding diathesis related to a profound, isolated and acquired prothrombin deficiency; evidence for a "lupus anticoagulant" was also demonstrated. This association of hypoprothrombinemia and "lupus anticoagulant", rarely reported, was previously considered to be rather specific for SLE. This case report demonstrates that these coagulation disorders may present as an acute form, in viral diseases of the child, with spontaneous and quick recovery. Specific characteristics of the biological coagulation defects, namely those related to the low factor II, are discussed.