To investigate the correlation between maternal and fetal angiotensin-converting enzyme gene insertion or deletion polymorphism and birth size, angiotensin-converting enzyme gene insertion or deletion polymorphisms of 470 Japanese pairs of mothers and infants were genotyped. The relationships between maternal and fetal angiotensin-converting enzyme gene insertion or deletion polymorphisms and birth sizes (weight, length, head circumference, and the incidence of small-for-gestational age status) were analyzed. No relationship between fetal angiotensin-converting enzyme genotype and birth size was observed. In contrast, the incidence of small-for-gestational age status in babies whose mothers had 1 or 2 deletion alleles was greater than that in babies whose mothers had 2 insertion alleles (7% vs 3%, P = 0.045). Maternal deletion allele of angiotensin-converting enzyme is associated with fetal growth restriction in the Japanese normal population.