Meiotic segregation of gonosomes from a 46,XY/47,XXY male was analysed by a three-colour fluorescence in situ hybridisation (FISH) procedure. This method allows the identification of hyperhaploid spermatozoa (with 24 chromosomes), diploid spermatozoa (with 46 chromosomes) and their meiotic origin (meiosis I or II). Alpha satellite DNA probes specific for chromosomes X, Y and 1 were observed on 27,097 sperm nuclei. The proportions of X- and Y-bearing sperm were estimated to 52.78% and 43.88%, respectively. Disomy (24,XX, 24,YY, 24,X or Y,+1) and diploidy (46,XX, 46,YY, 46,XY) frequencies were close to those obtained from control sperm, whereas the frequency of hyperhaploid 24,XY spermatozoa (2.09%) was significantly increased compared with controls (0.36%). These results support the hypothesis that a few 47,XXY germ cells would be able to complete meiosis and to produce mature spermatozoa.