Heights and weights were analysed for 636 twins who had been repeatedly measured from birth to 4 years. MZ twins were less concordant for birth weight than DZ twins, principally as a result of a few MZ pairs who exhibited very large differences. By one year of age, however, MZ twins had become more concordant for weight (Rmz = 0-87) while DZ twins moved further apart (Rdz = 0-55). For height, at birth, the MZ correlation was 0-58 while the DZ correlation was 0-82, but by 2 years, the MZ correlation reached 0-89 and the DZ correlation regressed to 0-58. The actual size differences within pairs followed a parallel course--for MZ twins, the differences became smaller following birth, whereas DZ differences became larger. The results are discussed in terms of (a) prenatal influences that differntially affect birth size within MZ pairs and DZ pairs, and (b) the rapid convergence of each twin on his genetic growth curve.