A placental lesion, characterized by fibrinoid and trophoblastic necrosis with massive infiltration of the intervillous space by mononuclear cells (massive chronic intervillositis, MCI), was observed in six cases, five with intrauterine growth retardation (IUGR) and one with sudden intrauterine fetal death. Four out of six had chronic villitis of unknown etiology (CVUE) associated with this lesion, and five had lesions of anchoring villitis. In three cases, acute atherosislike lesions in spiral arteries of parietal and/or basal decidua were observed. Massive deposits of IgM, a smaller amount of C3 and Clq, and slight deposits of IgG and IgA were found in these vessels. Neither mothers nor infants had any clinical or serological evidence of infection. Cases with MCI were compared with those having CVUE without intervillositis. Patients with MCI showed lower values of infant weight, infant length, and ponderal index than controls. However, cases with MCI group showed a higher incidence of IUGR. Placentas from the MCI group had a greater number of acute atherosislike lesions than controls. Massive chronic intervillositis may represent an extreme variant of villitis of unknown etiology. A maternal immunological aggression toward fetal tissues is proposed as pathophysiological mechanism, although a nondetermined placental infection cannot be excluded.