Phenotypic and clinical characteristics of aneuploidy of sex chromosomes in KBC Osijek Objectives: The objectives of the study were to determine the frequency of aneuploidies of sex chromosomes and investigate the association between the phenotype of the subjects with specific sex chromosome disorders, and compare them with data from the literature. Study Design: A cross-sectional study with historical data was conducted. Participants and Methods: The study included 31 patients who were being treated and monitored at the Medical Genetics and Metabolic Diseases Clinic, Department of Pediatrics, Clinical Hospital Center Osijek, over a ten-year period from January 1, 2012, to December 31, 2022. Results: Out of the 31 patients with sex chromosome aneuploidies, 11 (36 %) had Klinefelter syndrome, 6 (19 %) had Turner syndrome, XYY or androgen insensitivity syndrome, and 2 (7 %) had triple X syndrome. Family history was recorded in 11 (36 %) cases, and paternal and maternal age over 35 years was recorded in 8 (29 %) and 5 (16 %) cases, respectively. The median age of patients at diagnosis was 14 years, with the youngest patients having XYY syndrome and the oldest having Klinefelter syndrome. A variety of phenotypic characteristics were observed, some specific to certain sex chromosome disorders. Eight (26 %) patients had psychomotor developmental difficulties, and 6 (19 %) had language and speech development issues. Regarding intelligence, 9 (29 %) patients had borderline intellectual abilities, and one had mental retardation. Conclusion: An association was found between a family history burden and parental age over 35 years with an increased occurrence of sex chromosome aneuploidies. The most frequent disorder recorded is Klinefelter syndrome, and the diversity of phenotypic characteristics in sex chromosome aneuploidies is the reason for their lower recognition in the general population.