When searching the genomes of human, fly and worm for cases of exon duplication, we found that about 10% of all genes contain tandemly duplicated exons. In the course of the analyses, 2438 unannotated exons were identified that are not currently included in genome databases and that are likely to be functional. The vast majority of them are likely to be involved in mutually exclusive alternative splicing events. The common nature of recent exon duplication indicates that it might have a significant role in the fast evolution of eukaryotic genes. It also provides a general mechanism for the regulation of protein function.