To characterize the histological changes in malarial placentas and their relationship with parity and maternal and cord parasitemias, we conducted a histological study on 1,179 placentas from Ifakara, Tanzania, an area with intense and perennial malaria transmission. Immunohistochemical and quantitative studies for CD45, fibrin, and villous area were performed in 60 cases. Four hundred fifteen placentas (35.2%) showed parasites (active infections); in 303 of them, parasites co-existed with pigment covered by fibrin (chronic infections), and in 112 only parasites were detected (acute infections). Four hundred seventy-five cases (40.3%) showed hemozoin deposition without parasites (past infections). Of women with parasitized placentas, 46.3% did not show parasites in the peripheral blood. Basal membrane thickening (P = .002), fibrinoid necrosis (P = .004), and prominence of syncytial knots (P = .031) were associated with active malarial infection. No quantitative differences for perivillous fibrin deposition or villous area were found. The most significant association with active malarial infection was intervillous infiltration by mononuclear inflammatory cells (P < .001). Chronic infections were associated with the most severe changes, particularly intervillous mononuclear inflammation (OR, 28.7; 95% CI = 16.0 to 51.5, P< .001). Past infections showed only minimal differences with noninfected placentas. Primiparas showed chronic infections more frequently than multiparas (52% v 15%, P < .001). They also showed significantly higher placental parasitemias and intervillous inflammatory infiltrate. In conclusion, placental histology is more sensitive than peripheral blood examination in detecting malarial infection during pregnancy. Most malarial infections recover during pregnancy, leaving few residual changes in the placenta. Intervillous inflammation is the most frequent finding associated with malaria and is especially severe in primiparas, suggesting that mechanisms other than immunosuppression are responsible for the high susceptibility in this group.