First-borns are smaller than later-borns at birth. We investigated adult stature of families with three or more adult children, aged between 20 and 70 (mean 32) years. There was no significant correlation between size of the family and final adult stature of the siblings. We found a mean height reduction of first-borns compared to the mean of all sibs of -1.2 cm (P less than 0.02 using conventional analysis of variance, P less than 0.05 using nonparametric prediction configural frequency analysis). Males tend to increase in height with increasing birth order, whereas females do not beyond the second-born. These observations could not be explained by decreasing age of later-born sibs, nor by periods of malnutrition immediately after the war (1945-1948). Thus, we conclude that birth order is a factor that contributes significantly to the variance of adult height within sibships.