There are several dynamic influences on anterior fontanelle development in infants; among them, brain growth, dural attachments, suture development, and osteogenesis, It thus seems reasonable to hypothesize that variation in anterior fontanelle development between infants, related and unrelated, might have a significant genetic component. Anterior fontanelle size was quantitated by the method of Popich and Smith for 94 monozygotic (MZ) and 187 dizygotic (DZ) four-month-old twin pairs. The general model for estimating genetic variance from quantitative twin data was applied to MZ and DZ twins and then separately by chorion type. Since there were significant mean differences between blacks and whites, races were analyzed separately. The within-pair mean square estimates of genetic variance (GWT) were highly significant for both blacks (less than 0.02) and whites (P less than 0.002). Comparisons of means, total variances, and among-pair mean squares within races revealed no heterogeneity. There were also no significant chorion effects. Since the anterior fontanelle closes at around 1--1 1/2 years of age, it was evaluated at age one in 95 MZ and 194 DZ twin pairs as a qualitative trait - ie, open vs closed, concordance vs discordance. There were no significant differences in proband concordance rates between MZ and DZ twin pairs for either blacks (P greater than 0.5) or whites (P greater than 0.10). Again, there were no significant chorion effects. These data suggest that anterior fontanelle developmental variation has a significant genetic variance component at four months of age but not at one year. This finding may be related to the rapid brain growth witnessed between birth and eight to nine months of age.