<h4>Background</h4>Megalencephaly-capillary malformation (MCAP) is a rare overgrowth syndrome caused by postzygotic activating mutations in the <i>PIK3CA</i> gene.<h4>Aim</h4>To illustrate the benefits of gingival biopsy in the genetic diagnosis of overgrowth syndromes.<h4>Design</h4>Gingival biopsy was performed on a 13-year-old patient and a 16-year-old patient with MCAP and who suffered from periodontal disease. This evidence concerns the gene PIK3CA and overgrowth syndrome.