Genital abnormalities such as congenital uni/bilateral absence of the vas deferens are very rare in Klinefelter's syndrome. Here, we report three cases of Klinefelter's syndrome with unilateral absence of the vas deferens. All cases had small testicles, and unilateral vas deferentia were not palpable. Hormonal evaluations revealed hypergon-adotropism. One case had elevated prolactin level, and pituitary adenoma was detected by magnetic resonance imaging. All cases were diagnosed as Klinefelter's syndrome (one of them had mosaicism) cytogenetically, and some CFTR gene mutations were detected. To our knowledge, this is the first case series of both conditions existing simultaneously.