Sister chromatid exchanges (SCE) were analyzed in the cord and postnatal blood of controlled groups of low and high birth weight infants to detect possible associations between abnormal birth weight and SCE frequency. Structural chromosome aberration rates had previously been evaluated for all infants, and possible correlations between aberration and SCE rates were sought. No correlation was found between neonatal or postnatal SCE frequency and birthweight, nor was there evidence of association of chromosome aberration rates with SCE frequency. In all groups of infants, however, mean postnatal SCE frequencies were significantly lower than mean neonatal SCE frequencies.