['Studies that use a murine model of antiphospholipid syndrome have demonstrated a critical role for complement activation that leads to fetal and placental injury in the presence of antiphospholipid antibodies (APAs). We examined the placentas of patients with APAs to demonstrate a similar association with tissue injury in humans.', 'Immunohistochemical analyses with the use of antibodies to the complement products C4d, C3b, and C5b-9 were performed on paraffin-embedded tissue sections of placentas from 47 patients with APAs and 23 normal control patients.', 'We found evidence of increased complement deposition in the trophoblast cytoplasm (C4d and C3b), trophoblastic cell and basement membrane (C4d), and extravillous trophoblasts (C4d) of patients with APAs, compared with control patients. We report a correlation between placental pathologic features and complement deposition (C4d) in the trophoblastic cytoplasm, cell membrane, and basement membrane.'] Is excessive complement activation associated with placental injury in patients with antiphospholipid antibodies?