It has been suggested that oral supplements of folic acid interfere with the intestinal absorption of zinc and may have toxic side effects. The concentrations of Zn and folate in blood were monitored in a group of women with cervical dysplasia randomly assigned to receive 10 mg/d of either folic acid (pteroylglutamic acid) or ascorbate. Fifty subjects were evaluated after 2 mo; 21 of the same subjects were evaluated again after 4 mo. No untoward clinical effects were observed. Significant elevation of erythrocyte folate above the baseline value was observed in the supplemented group but not in the placebo group (p less than 0.001). The concentration of Zn in plasma and erythrocytes did not change significantly in either the folate-treated or placebo groups after 2 and 4 mo. It is concluded that carefully controlled clinical intervention trials of this type do not impose a risk of depleting the concentration of Zn in erythrocytes and plasma.