['The aim was to investigate the growth of monozygotic twins with discordant birth weights and the predictive value of birth-weight, birth-length, and cord-blood concentration of growth factors on their catch-up growth up to the age of 4 yr.', 'Twenty-five monozygotic twin-pairs [14 with an intertwin birth-weight sd score (SDS) difference>1] were studied at birth and at 4 yr of age. In all pairs, several parameters including IGF-I were analyzed in cord blood, and in 20 pairs parameters were analyzed again at 4 yr of age. Intertwin differences (Δ) in birth weight, birth length, and growth at 3, 6, 12, 24, and 48 months were correlated with Δ of the parameters analyzed.', 'We found a reduction of Δ height SDS from birth to 4 yr, with the main catch-up occurring during the first year, but only a slight, statistically insignificant reduction of Δ body mass index SDS during the observation period. Correlation coefficients were used to identify factors predicting postnatal catch-up growth. Both birth-weight difference (r=0.653; P=0.001) and Δ IGF-I in cord blood (r=0.613; P=0.007) were of similar predictive value. Variance analysis showed no significant difference in individual impact between these two parameters, although both correlated strongly with actual height.'] Is insulin-like growth factor-I in cord blood predictive of catch-up growth in monozygotic twins with discordant growth?