We analyzed family structure, genetic patterns, epidemiology, and vitamin usage in a series of families with multiple cases of spina bifida (familial SB). Among 6,491 individuals ascertained in 72 families with familial SB, we identified 180 patients--85 males and 95 females. The number of collateral cases on the maternal side (49 of 3,588), analyzed by category of kinship, were significantly higher than those on the paternal side (16 of 2,903) (p = 0.0002). Genomic imprinting or a partial mitochondrial contribution are possible mechanisms for this maternal effect. The proportion of US-born SB families reporting some Irish ancestry (49%, 34 of 70) or some German ancestry (50%, 35 of 70) were significantly higher than those for the US population at large. In contrast, the proportion of families reporting some African-American ancestry (1%, 1 of 70) was significantly lower. The elevated proportions of families with Irish and German ancestry, the high frequency of SB in Northern Ireland and in certain regions of Germany, the reduced proportion of families with African-American ancestry, and the lower prevalence of SB in African-Americans all suggest a genetic contribution to the etiology of the disorder. In our study, the proportion of mothers who used supplemental vitamins during the periconceptional period (29%, 47 of 163) was not significantly different from that in the US population at large.