Diagnosis of Klinefelter's syndrome relies on raised gonadotropin levels in serum, azoospermia, determination of sex chromatin in oral swabs and finally chromosome analysis in leukocyte cell culture. By this method the numerical chromosome aberration with a 47, XXY karyotype can be detected. However, diagnosis can be accelerated by demonstration of RNA expression of an X‐linked gene, which serves as a marker for inactivation of the second and any further extra X chromosome in the cell. This so‐called X‐inactive‐specific transcript (XIST) is transcribed exclusively from the inactive X chromosome. RNA was isolated both from Ficoll‐prepared peripheral blood leukocytes and from total EDTA blood of Klinefelter patients and control persons. RNA was reverse transcribed and finally detected by the polymerase chain reaction (PCR) with XIST‐specific sequences. The pyruvate dehydrogenase gene was used as a control gene for successful RNA preparation and reverse transcription. XIST transcripts could be detected in all blood samples from Klinefelter patients (n = 15, karyotype 47, XXY) and female persons (n = 3). Fertile men (n = 5) were negative for this transcript in peripheral blood. Thus, diagnosis of Klinefelter's syndrome can be accelerated without loss of sensitivity and specificity by detection of XIST expression in peripheral blood leukocytes.