Somatic mosaicism in genetic disease generally results from a de novo deleterious mutation during embryogenesis. We now describe a somatic mosaicism due to the unusual mechanism of in vivo reversion to normal of an inherited mutation. The propositus was an adenosine deaminase–deficient (ADA−) child with progressive clinical improvement and unexpectedly mild biochemical and immunologic abnormalities. Mosaicism due to reversion was evidenced by absence of a maternally transmitted deleterious mutation in 13/15 authenticated B cell lines and in 17% of single alleles cloned from blood DMA, despite retention of a maternal ‘private’ ADA polymorphism linked to the mutation. Establishment of significant somatic mosaicism following reversion to normal could modify any disorder in which revertant cells have a selective advantage.