We conducted this study to determine whether antiprothrombin antibody (aPT) [to prothrombin (PT) alone or PT/phosphatidyl serine (PS) complex] actually existed in patients with lupus anticoagulant (LA) and/or anticardiolipin antibody (aCL). aPT to PT alone was positive in 2/7 LA-positive (29%) and 3/7 LA/aCL-positive (43%) patients. aPT to PT/PS complex was positive in 4/7 LA-positive (57%) and 4/7 LA/aCL-positive (57%) patients in the presence of Ca2+. However, none of the aCL-positive patients without LA or the LA/aCL-negative patients were positive for aPT and aPT/PS. Thus, we confirmed the existence of aPT and aPT/PS specifically among LA-positive patients. However, the clinicopathological significance of aPT and aPT/PS in this clinical setting is yet to be clarified.