Mutations in recombination activating genes (RAG) 1 and 2 have been found to cause Omenn syndrome (OS), a severe combined immunodeficiency (SCID) with a peculiar phenotype. Here we report the prenatal diagnosis performed in three OS patients. Mutations were detected in the probands as well as in their parents by genomic sequencing of the complete coding regions of both RAG 1 and RAG 2, which are contained in a single exon. All the three probands had RAG 1 mutations in both alleles, at least one of which was a missense substitution. Of the three fetuses tested, one had a wild type sequence on both alleles, while the other two had one mutated allele. None of the three patients were predicted to be affected and this was confirmed at birth. Detection of RAG genes mutations on fetal samples by direct sequencing is an easy and effective way to investigate fetuses from families affected with RAG-dependent SCID and OS families affected by RAG-dependent SCID and OS.