The present case report describes the oral features of tetraploid/diploid mosaicism. An 11-year-old boy with severe periodontal destruction is presented in this report. He was examined clinically, radiologically, immunologically, and genetically. Significant edema of the gingiva, severe sulcular bleeding on probing and mobility of many teeth were detected on intraoral examination. There was severe generalized maxillary and mandibular bone loss as determined by radiological examination. He was diagnosed as generalized aggressive periodontitis. The cytogenetic examination revealed 92,XXYY (4%) / 46,XY (96%) karyotype indicating tetraploid/diploid mosaicism. Nonsurgical periodontal therapy was applied and he is currently under a routine follow-up period. In this report, the oral characteristics of tetraploid/diploid mosaicism are described. Dental practitioners should see these patients in some distinct periods, because tetraploid/diploid mosaicism subjects may have aggressive periodontitis.