BACKGROUND: Helminth infections during pregnancy may be associated with adverse outcomes, including maternal anemia, low birth weight, and perinatal mortality. Deworming during pregnancy has therefore been strongly advocated, but its benefits have not been rigorously evaluated. METHODS: In Entebbe, Uganda, 2507 pregnant women were recruited to a randomized, double-blind, placebo-controlled trial investigating albendazole and praziquantel in a 2 x 2 factorial design [ISRCTN32849447]. Hematinics and sulphadoxine-pyrimethamine for presumptive treatment of malaria were provided routinely. Maternal and perinatal outcomes were recorded. Analyses were by intention to treat. RESULTS: At enrollment, 68% of women had helminths, 45% had hookworm, 18% had Schistosoma mansoni infection; 40% were anemic (hemoglobin level, <11.2 g/dL). At delivery, 35% were anaemic; there was no overall effect of albendazole (odds ratio [OR], 0.95; 95% confidence interval [CI], 0.79-1.15) or praziquantel (OR, 1.00; 95% CI, 0.83-1.21) on maternal anemia, but there was a suggestion of benefit of albendazole among women with moderate to heavy hookworm (OR, 0.45; 95% CI, 0.21-0.98; P=.15 for interaction). There was no effect of either anthelminthic treatment on mean birth weight (difference in mean associated with albendazole: -0.00 kg; 95% CI, -0.05 to 0.04 kg; difference in mean associated with praziquantel: -0.01 kg; 95% CI, -0.05 to 0.04 kg) or on proportion of low birth weight. Anthelminthic use during pregnancy showed no effect on perinatal mortality or congenital anomalies. CONCLUSIONS: In our study area, where helminth prevalence was high but infection intensity was low, there was no overall effect of anthelminthic use during pregnancy on maternal anemia, birth weight, perinatal mortality, or congenital anomalies. The possible benefit of albendazole against anemia in pregnant women with heavy hookworm infection warrants further investigation.