BACKGROUND Periconceptional supplementation with folic acid has been shown to decrease the occurrence of neural tube defects (NTD). A daily supplement of 400 micro g folic acid from the month before pregnancy and the first 2 to 3 months of pregnancy is recommended in Norway. We wanted to evaluate the use of folic acid supplements among pregnant women in Oslo, and to explore demographic factors that were associated with periconceptional intake. METHODS All women attending antenatal ultrasound screening at the two main obstetric departments in Oslo during 3 months in 2001 were invited to answer questions regarding folic acid supplementation before and during their present pregnancy. RESULTS A total of 1541 women (91, 5%) participated in the study, of which 26% were nonwestern immigrants. Only 17% of all women had started the folic acid supplementation preconceptionally (22% of the nonimmigrant vs. 2% of the immigrant population). Folic acid had been used by 58% at some time during the present pregnancy (73% of the nonimmigrant vs. 19% of the immigrant population). Regression analysis demonstrated that the difference in periconceptional use of folic acid between the two population groups could not be explained by registered demographic factors other than immigrant status. CONCLUSIONS Most pregnant women in Oslo do not use folic acid as recommended and a dramatically low usage is seen among the immigrant population, indicating that more widespread information is necessary. Furthermore, fortification of cereals with folic acid should be considered to secure a sufficient intake of folic acid in all fertile women.