Background: Klinefelter’s syndrome (KS) is the presence of one or more supernumerary X chromosomes. The aim was the investigation of the cytogenetic variant in men with KS, for the assessment of neurological phenotypes. Material and methods: Were investigated 98 men with infertility, having as selection criteria, lack of sperm in the ejaculate, elevated values of Follicle-stimulating hormone (FSH), Luteinizing Hormone (LH), and the following phenotypic aspects: small testes, hypogonadism, cryptorchidism, waist high and disproportionate, gynecomastia, mental retardation, psychosocial problems. Karyotyping was performed according to standard methods G-banding. Results: The most common cytogenetic variant diagnosed in 25 (25.5%; [95 CI 21.1 – 29.9], p = 0.05) patients with SK was homogeneous free trisomy 47, XXY (22 cases – 88%), followed by: mosaic form (47, XXY / 46, XY: 1 case), polysomies X – Y variants (48, XXYY and 49, XXXXY: 2 cases). In the patients with variant 47, XXY the classical and mosaic forms showed a mild to moderate mental retardation (36.0%; [95 CI 26.4 – 45.6], p = 0.05), language disorders with cognitive-verbal retardation (48.0%; [95 CI 38.01 – 57.99], p = 0.05), slow motor development (20.0%; [95 CI 12.0 – 28.0], p = 0.05), coordination disorders (8.0%; [95 CI 2.57 – 13.43], p = 0.05), immature behavior (60.0%; [95 CI 50.2 – 69.8], p = 0.05). In patients with variants 48, XXYY and 49, XXXXY, moderate to severe mental retardation (50.0%; [95 CI 14.64 – 85.36], p = 0.05), severe cognitive-verbal retardation (50.0%; [95 CI 14.64 – 85.36], p = 0.05), behavioral problems and life-threatening problems were found in 100%. Conclusions: The cytogenetic variant of KS depends on the number of supernumerary X chromosomes, being determinant in the characteristic of neurological pheno