Tetrasomy 18p is an extremely rare chromosomal disorder that usually arises as a de novo mutation (a random error in early development). The article describes the case of a 17-year-old girl with mosaic tetrasomy 18p, which represents the first reported case of this type in an elderly patient. The diagnosis was confirmed by G-banding, C-banding and CNV-seq analysis, which revealed a duplication of 14.86 Mb in the 18p11.32-p11.21 region with a copy number of 3.5. The patient showed mental retardation, oligomenorrhea (dilute menstrual bleeding), and characteristic physical features, including the absence of fingerprints on her palms. Genetic testing of the patient's parents and three sisters did not reveal any chromosomal abnormalities, which confirms the de novo origin of the disorder. The authors conclude that oligomenorrhea and the absence of palm prints may represent new characteristics of tetrasomy 18p. The combination of traditional cytogenetic methods and modern sequencing enabled early diagnosis and personalized treatment of this rare disease.