We present a case of mosaic 5p tetrasomy. The mosaicism 46,XX/47,XX,+i(5p) was found at different ratios in blood lymphocytes, skin fibroblasts, and chondrocytes. The origin of the extra isochromosome was confirmed by FISH. The clinical picture corresponds to that described in trisomy 5p patients, although it was more severe than the two previously reported cases of mosaic 5p tetrasomy. No correlation between clinical severity and proportion of tetrasomic cells in blood or fibroblasts was found in these cases.