In an effort to explain the causes of variation in birth weight within and between families, birth weights of 13,970 sons and daughters of monozygotic (MZ) and dizygotic (DZ) twins were analysed. The sample included birth weights of halfsibs and cousins related either through females or males, permitting the distinction between maternal and fetal genetic effects. Models of genetic and environmental variances were fitted to mean squares from analysis of variance. The results indicate that more than 50% of the total variation in birth weight is caused by variation in fetal genes, and that less than 20% is caused by variation in maternal genes. The remaining variance (20-30%) could be explained by random environmental effects. No certain effects were found of family-specific environment or of interactions between fetal and maternal genes.