Background Food fortification is one approach for addressing anemia, but information on program effectiveness is limited. Objective We evaluated the impact of Costa Rica’s fortification program on anemia in women aged 15–45 y and children aged 1–7 y. Design Reduced iron, an ineffective fortificant, was replaced by ferrous fumarate in wheat flour in 2002, and ferrous bisglycinate was added to maize flour in 1999 and to liquid and powdered milk in 2001. We used a one-group pretest-posttest design and national survey data from 1996 (baseline; 910 women, 965 children) and 2008–2009 (endline; 863 women, 403 children) to assess changes in iron deficiency (children only) and anemia. Data were also available for sentinel sites (1 urban, 1 rural) for 1999–2000 (405 women, 404 children) and 2008–2009 (474 women, 195 children), including 24-h recall data in children. Monitoring of fortification levels was routine. Results Foods were fortified as mandated. Fortification provided about one-half the estimated average requirement for iron in children, mostly and equally through wheat flour and milk. Anemia was reduced in children and women in national and sentinel site comparisons. At the national level, anemia declined in children from 19.3% (95% CI: 16.8%, 21.8%) to 4.0% (95% CI: 2.1%, 5.9%) and in women from 18.4% (95% CI: 15.8%, 20.9%) to 10.2% (95% CI: 8.2%, 12.2%). In children, iron deficiency declined from 26.9% (95% CI: 21.1%, 32.7%) to 6.8% (95% CI: 4.2%, 9.3%), and iron deficiency anemia, which was 6.2% (95% CI: 3.0%, 9.3%) at baseline, could no longer be detected at the endline. Conclusions A plausible impact pathway suggests that fortification improved iron status and reduced anemia. Although unlikely in the Costa Rican context, other explanations cannot be excluded in a pre/post comparison.