Among the factors implicated in the heterogeneous etiology of neural tube defects (NTDs) is the trace element zinc (Zn). In a case-control study, we collected midtrimester maternal toenail samples for multiple trace element analyses, including Zn, which were assayed by neutron activation analysis. We studied 17 women with NTD offspring and 1,787 controls. The crude OR for NTD comparing Zn values > normal range to normal Zn values was 3.2 (95% CI 1.1,9.7). These results were not materially affected when adjustment was made for folic acid supplementation. An overall increased risk for NTD associated with increasing toenail Zn was also evident. A matched subset of 17 cases and 73 controls yielded a crude OR of 3.1 (95% CI 0.9,10.3) when cases with elevated Zn (greater-than-or-equal-to 120 ppm) were compared to those with normal Zn. Matched analyses controlling for folic acid supplements, family history of NTD, assay batch. age of mother and year of delivery yielded an OR of 5.0 (95% CI 1.1,21.6). This study reveals an association between increased toenail Zn in the second trimester of pregnancy and the risk of having a child with an NTD. Whether Zn sequestration has resulted in relative Zn deficiency at the site of neural tube closure remains uncertain.