The objective was to evaluate the association between prophylactic iron supplementation and prematurity and low birth weight. Pregnant women 20 years and older with 21 to 28 weeks of gestational age were enrolled consecutively in prenatal services in the Unified National Health System in six Brazilian State capitals between 1991 and 1995. Socio-demographic data and information on iron supplementation up until the 28th gestational week were obtained by means of an interview. Outcomes and other variables were collected from medical records. Anemia was present in 31.3% of the 3,865 women. Among anemic women (hemoglobin < 11.0g/dL), 29.8% were taking iron supplements, as compared to 16.7% among non-anemic women (hemoglobin > or = 11.0g/dL). After adjusting for potential confounders, iron was not associated with prematurity (OR = 0.88; 95%CI: 0.73-1.07), low birth weight (OR = 0.99; 95%CI: 0.75-1.31), or very low birth weight (OR = 0.58; 95%CI: 0.29-1.13). The results suggest that iron supplementation up to the 28th gestational week does not reduce the risk of prematurity, low birth weight, or very low birth weight.