objective: dietary intake of methyl donors such as folic acid prevents neural tube defects (ntd), but recent studies showed that it might also favor the development of asthma. in this work a possible ecological association between ntd and asthma was explored. material and methods: data bases from mexico and the united states (us) were reviewed to obtain information about geographical distribution (by state) and temporal trends (by year) of ntd and asthma. results: those states with the lowest frequency of ntd had the highest frequency of asthma, both in mexico (rs=-0.48, p=0.005) and us (rs=-0.39, p=0.005). temporal trends also showed an inverse correlation in mexico (1997-2007, rs=-0.73, p=0.01) and us (1979-1998, rs=-0.91, p<0.001). conclusions: in both countries the frequency of asthma inversely correlated with the frequency of ntd, both in geographical distribution and annual trends, giving support to the possibility that methyl donors intake in diet or supplements is influencing the asthma frequency.