Janerich, D. T. (Birth Defects Institute, N. Y. State Dept. of Health, Albany, N. Y. 12208). Maternal age and spina bifida: longitudinal versus cross-sectional analysis. Am J Epidemiol 96: 389–395, 1972.—This study examines the relationship between maternal age and the risk of delivering a child with spina bifida, through use of data from New York State birth records. In most previous studies this relationship has been described through longitudinal interpretation of cross-sectional data. It is observed that, where the relationship does not remain constant over time, this practice can lead to erroneous appraisals of the risk. Our data show that the cross-sectional and longitudinal approaches produce differing results, and the difference is probably not explained by changes in the risk associated with the time of birth of the affected child. The results essentially agree with an earlier, more limited study, on the relationship between anencephaly and maternal age. The findings raise two important epidemiologic questions. First, what methods can be used to assess the relationship between maternal age and risk of defective offspring, when the risk appears to change with both time and maternal age? Second, if the relative risk, is cohort related, what type of role might environmental factors play in determining that risk? We have attempted some answers to these questions, but reappraisal and further study of this relationship are needed.