We studied the longitudinal association of birth order and birth weight in two series of very large sibships, each consisting of at least seven children, and compared the findings with those based on analysis of cross-sectional data from a large population-based survey, the Jerusalem Perinatal Study. The birth weights of the cross-sectional sample were adjusted by multiple linear regression for a number of factors known to confound cross-sectional studies, including maternal age, education, marital status, religion, smoking, height and prepregnant weight, gestational age, and sex of the newborn. Birth weight increased with increasing birth order in both adjusted cross-sectional and socioeconomically homogeneous longitudinal data.