Klinefelter syndrome is the first human sex chromosomal abnormality to be reported. The majority of Klinefelter syndrome patients have the XXY karyotype. Approximately 15% of Klinefelter patients, however, are mosaics with variable phenotypes. Among the variant Klinefelter genotypes are such karyotypes as XY/XXY and XX/XXY. The variation in phenotypes most likely depends on the number of abnormal cells and their location in body tissues. In this paper we report the case of a 42-year-old patient with Klinefelter syndrome and a rare variant mosaic XXY/XX karyotype initially identified by GTG-banding. This was confirmed by fluorescence in situ hybridization (FISH) using a dual-color X/Y probe. The patient presented with erectile dysfunction and few other physical findings. Thus, this case illustrates a rare variant of Klinefelter syndrome with a relatively mild phenotype. It also illustrates the utility of FISH as an adjunct to conventional cytogenetics in assessing the chromosome copy number in each cell line of a mosaic. In our case, FISH also detected the presence of a small population of cells with the XY karyotype not previously detected in the initial 30-cell GTG-banding analysis. Thus, through a combination of GTG-banding and FISH, the patient was determined to be an XXY/XX/XY mosaic. Given that most individuals with Klinefelter syndrome are infertile, and that these individuals may wish to reproduce with the aid of modern reproductive technology, such as testicular fine needle aspiration and intracytoplasmic sperm injection, it is important that accurate estimation of the frequency of abnormal cells be obtained for accurate risk estimation and genetic counseling, as recent studies in patients with mosaic Klinefelter syndrome revealed that germ cells with sex chromosomal abnormalities were nevertheless capable of completing meiosis.