OBJECTIVE: To evaluate the effectiveness of three regimens employing ferrous sulfate to treat pregnant women with anemia. METHODS: The study was carried out at the Women's Health Center of the Pernambuco Institute of Maternal and Child Health in the city of Recife, Pernambuco, Brazil, from May 2000 to December 2001. A randomized clinical trial with blinded laboratory analysis was conducted. Iron (60 mg) was administered as 300-mg ferrous sulfate tablets. The women were allocated to three treatment groups, according to the frequency of ingesting the tablets: once a week (48 women), twice a week (53 women), and once a day (49 women). The groups were compared for the values for hemoglobin (Hb) concentration, mean corpuscular volume, and ferritin before and after the treatment. RESULTS: Before the intervention, the groups were homogeneous. They had the following mean (+/- standard deviation) concentrations of hemoglobin: 10.2 +/- 0.5 g/dL for the group receiving iron once a week, 10.2 +/- 0.6 g/dL for the group receiving iron twice a week, and 10.1 +/- 0.6 g/dL for the group receiving iron once a day. The means of corpuscular volume were, respectively: 88.5 +/- 5.0 fL, 87.6 +/- 5.9 fL, and 88.7 +/- 5.1 fL. The respective medians for ferritin were 30.2 ng/mL, 37.1 ng/mL, and 52.9 ng/mL. The cure rate (Hb > 11 g/dL) was 27% in the patients treated once a week, 34% in those treated twice a week, and 47% in the women treated daily. Treatment failure (hemoglobin < 10 g/dL) was seen in 41.6%, 13.2%, and 2.0% of the patients in the respective groups. Interruption of treatment due to diarrhea or epigastric pain occurred only among the patients treated daily. CONCLUSION: The regimen with iron administered daily is still the best option for treating anemia. However, treatment with ferrous sulfate administered twice a week is an alternative for patients who are unable to adhere to daily treatment.