ABSTRACT: Lesions of acute atherosis in 23 placentas from pregnancies complicated by fetal growth retardation (less than tenth percentile) were studied by an immunoperoxidase staining. Twelve of them were from normotensive pregnancies, ten complicated by pregnancy induced hypertension, and the remaining associated with systemic lupus erythematosus. Twentythree placentas with physiological changes of the spiral arteries corresponding to normal pregnancies served as controls. Six of the cases complicated by fetal growth retardation showed absence of physiological changes in basal decidua. Massive intramural granular deposits of IgM, a smaller amount of C3 and C1q, and slight deposits of IgA and IgG were found in vessels with acute atherosis. No intramural deposition of immunoglobulins or complement was observed in vessels with or without physiological changes. The presence of granular deposits of immunoglobulins and complement within the vessel walls with acute atherosis may be related to an immunological disorder, probably mediated by immune complexes.