ABSTRACT: We have investigated the presence and clinical implications of maternal vascular lesions and chronic villitis of unknown etiology (CVUE) in 18 placentas of 15 mothers with several autoimmune diseases (AD), including, for the first time, idiopathic thrombocytopenic purpura, autoimmune thyroid diseases, and multiple sclerosis. The group with AD had significantly more maternal vascular lesions and CVUE than the control group. We did not find lesions that could be attributed to any of the diseases in particular. The histopathologic picture was similar in these diseases, although there appears to be a spectrum in severity. Placental vascular damage with deposits of IgM, C3, and C1q was more prominent in systemic lupus erythematosus and in a patient with systemic sclerosis. In both of these diseases but not in the other conditions, these lesions were related to poor fetal outcome. Although the precise role of each of these autoimmune diseases in pregnancy and fetal outcome remains to be established, there appears to be at least one link between them represented by the presence of severe acute atherosis and heavy granular vascular deposits of IgM, C3, and C1q associated in some with poor fetal outcome. The role of CVUE remains speculative.