We describe a 1-year-old boy with a rare de novo 46,XY/47,XY, + i(5p) mosaicism (ratios 28/3 in peripheral blood lymphocytes and 2/12 in skin fibroblasts). The boy, born after a pregnancy of 34 weeks, had lung hypoplasia, persistent hypotonia, and postnatal growth failure. Craniofacial anomalies were also present. His clinical manifestations correspond to those described in trisomy 5p patients. Prenatal diagnosis on maternal age indication had shown normal male chromosomes in 16 cells in the short term culture of a chorionic villus sampling. Retrospectively, 1 out of 217 cells in this culture showed the i(5p). Several mechanisms could have resulted in the formation of this 46/47, + i(5p) mosaic. Postzygotic local incorrect ligation during chromatid replication, followed by a second replication offers an attractive model on theoretical grounds since it needs only one step to explain both isochromosome formation and mosaicism. Differences between the various tissues in selection pressure on cells with the isochromosome might explain the different ratios of mosaicism found.