Objective. - To identify predictors of red cell folate level in women attempting to become pregnant. Design. - Cohort study. Setting. - A health maintenance organization serving the Minneapolis-St Paul, Minn, area. Participants. - A total of 189 healthy, primarily white women aged 22 to 35 years enrolled in the Diana Project, a population-based prospective study of preconceptional and prenatal risks to reproductive outcomes. The sample represents 189 of 219 enrolled women who were sequentially selected from the total Diana Project sample to receive additional laboratory analyses. Main Outcome Measure. - Red cell folate level. Results. - Folic acid supplements, folic acid intake from fortified cereals, vitamin C supplements, and serum zinc level (inverse) were found to predict red cell folate levels. Previous research has shown that red cell folate levels higher than 906 nmol/L (400 ng/mL) may be optimal for the prevention of folate-responsive neural tube defects. For folic acid supplement users, folate intakes of 450 μg per day and higher corresponded to these protective levels of red cell folate. In nonusers of supplements, intakes of more than 500 μg of folate per day from foods and folic acid-fortified cereals may be needed to attain red cell folate levels higher than 906 nmol/L (400 ng/mL). Red cell folate levels higher than 906 nmol/L (400 ng/mL) were primarily found in women who took folic acid supplements. Only 1 in 4 women had red cell folate levels higher than 906 nmol/L (400 ng/mL), while 1 in 8 had red celt folate levels indicative of a negative folate balance. Addition of a daily, 400-μg folic acid supplement to the usual diet would result in red cell folate levels over 906 nmol/L (400 ng/mL) in a majority of women in this study. Conclusions. - Supplementation of diets of women of childbearing potential with 400 μg of folic acid per day would effectively raise red cell folate to levels associated with a low risk of folate-responsive neural tube defects. Protective levels of red cell folate may also be obtained by ample consumption of vegetables, fruits, and folic acid-fortified breakfast cereals. Efforts to increase folic acid supplement use and folate consumption among women of childbearing potential must go beyond fortification of refined cereal and grain products and reach women within all educational and income groups.