The cause of Sotos syndrome is unknown but it usually occurs sporadically. Recent studies have shown no evidence of uniparental disomy. One set of concordant monozygotic twins has been reported. We have identified the Sotos syndrome in one of two 5-year-old male monozygotic twins. Our finding of discordance in these identical twins suggests that a postconceptual mutation, or epigenetic change and/or an environmental factor may be involved in the cause of Sotos syndrome.