This report describes a patient not suspected of having Klinefelter syndrome during life but diagnosed with it following postmortem examination using fluorescent in situ hybridization (FISH) for sex chromosomes and hormone serum analysis. A 49-year-old Japanese man had a history of nephrosis, heavy alcohol consumption, diabetes mellitus, and liver cirrhosis and had been undergoing dialysis for 10 years. He died of ruptured esophageal varices. Autopsy revealed hypogonadism, suggesting Klinefelter syndrome. This was confirmed by FISH, which showed a mosaic 46XY, 47XXY karyotype, and by serum analysis, which revealed high luteinizing hormone and follicle-stimulating hormone and low testosterone levels. Autopsy also revealed a nodular, bilateral, testicular Leydig cell hyperplasia. This report illustrates the value of postmortem laboratory investigations, particularly FISH for sex chromosomes and serum hormone analysis, for the demonstration of clinically uncertain or "occult" Klinefelter syndrome.