Background: Community iron supplementation programmes for pregnant women have lacked effectiveness, partly because of low compliance. Objective: To determine factors that influence compliance among pregnant women in Senegal. Design: Two hundred and twenty-one pregnant women, recruited from six health centres in Dakar during their first prenatal visit, were randomly assigned to receive either a prescription to purchase iron/folic acid tablets (control, n 5 112) to be taken daily, according to official policy, or to receive free tablets (treatment, n 5 109). Compliance was assessed 20 weeks after enrolment through interviews and pill count. Women with low or high compliance (,70% or $70%) were asked to explain what influenced their adherence to supplementation. Results: Overall compliance was 69%; it was significantly higher in the treatment than in the control group (86% vs. 48%; P , 0.0001). Women with high compliance (58%) were motivated by: (1) the perception of improved health upon taking the tablets (treatment 5 24%, control 5 10%); (2) the insistence by midwives that they take the tablets; and (3) the mention that the tablets would improve health. Women with low compliance (42%) reported: (1) the experience of side-effects that they associated with the tablets (treatment 5 13%, control 5 14%); (2) misunderstanding that they needed to continue taking the tablets throughout pregnancy (treatment 5 0%, control 5 18%); and (3) forgetfulness. Conclusion: Compliance with iron/folic acid supplementation in Senegal can be increased by providing women with clear instructions about tablet intake and educating them on the health benefits of the tablets.