A 39 year old male with primary infertility was diagnosed as having Klinefelter syndrome by conventional cytogenetic analysis, which also showed an abnormal chromosome 12. Fluorescence in situ hybridisation (FISH) analysis of the aberrant chromosome using a 12 specific centromeric probe showed a break in the alphoid repeats followed by an inversion within the short arm, resulting in a pseudodicentric chromosome. Further FISH analyses using telomeric and subtelomeric probes showed that the other breakpoint was in the subtelomeric region of the short arm. The karyotype is designated 47,XXY,inv(12)(p10p13.3). To our knowledge this is the first report of a case of "centric inversion".