During September 1987 to July 1989, in Malawi, clinical investigators enrolled 2946 pregnant women into a chemoprophylaxis study at their first prenatal care visit (mean, 5.6 months) at 4 rural sites in Mangochi District. They prescribed 1 of 3 chloroquine regimens to the women and followed them through delivery. The investigators measured Plasmodium falciparum parasitemia at enrollment, monthly thereafter, at delivery, and 2-6 months postpartum. For hospitalized infants, they measured parasitemia in the placenta and in the umbilical cord blood of the newborn. They also aimed to examine the association between HIV infection and malaria in pregnant women. 152 (5.5%) of the 2781 women for whom HIV test results and malaria blood smear examinations were available had confirmed HIV infection. Malaria parasitemia stood at 42% at enrollment and 19.1% at delivery. At enrollment, HIV-positive women had a higher malaria parasite prevalence rate than HIV-negative women (54.4% vs. 41.7%; relative risk [RR] = 1.31; p = 0.002). They also had a higher geometric mean density of parasitemia (1558 vs. 670/sq mm; p 0.0005). The parasite pattern was similar at delivery (34.7% vs. 18.2% [RR = 1.91] and 1589 vs. 373/sq mm, respectively; p 0.0005). The placenta of infants born in the hospital to HIV-positive mothers also had a higher prevalence of malaria parasites than those born in the hospital to HIV-negative mothers (38.2% vs. 22.5%; RR = 1.7; p = 0.0003). The prevalence of umbilical cord blood malaria infection was higher in infants born in the hospital to HIV-positive mothers than their counterparts (25.5% vs. 6.8%; RR = 3.76). At 2-6 months postpartum, the prevalence and density of malaria parasitemia rate did not differ significantly by HIV status. Parasitemia prevalence and density were higher in multigravida HIV-positive women than HIV-negative women but were similar in primigravid HIV-positive and HIV-negative women. These findings suggest that HIV infection reduces a pregnant woman's capacity to control P. falciparum parasitemia and placental and newborn infection.