Folate metabolism and the effects of folic acid on chromosome stability were studied in four related patients with the fragile X syndrome. In three adults, uptake and subsequent utilization of folate compounds for conversion of deoxyuridylate to thymidylate by marrow cells and stimulated lymphocytes, and the affinity and maximal transport velocity of erythrocyte membrane carriers, were normal. Numbers of sister chromatid exchanges and double-stranded DNA breaks were comparable in cells from patients and control subjects, but both were increased after incubation in folate-deficient media. In vitro expression of the fragile site was strikingly reduced by oral folate therapy. It is concluded that the folate-sensitive chromosomal defect in this syndrome is limited to a specific site, Xq28, and there is no generalized tendency to frequent DNA breaks or recombination. Although expression was modified by folic acid treatment in the patients, no consistent abnormality of folate metabolism could be identified.