Two cases of a nonfluorescent Y (Ynf) chromosome were diagnosed: one in a male, the other in a female. Both had similar complex mosaic chromosome constitutions with a 45,X cell line. DNA studies were applied in both cases for verification of the cytogenetic diagnosis. The results on the two patients were compared with data obtained from seven healthy men (46,XY), three healthy women (46,XX), two females with 46,XY karyotype, and from cell lines with 49,XXXXY and 48,XXXX chromosome constitution. The highly repetitive Y-specific DNA sequences located in the heterochromatic region of the long arm were absent in these patients. Differences in the composition of the euchromatic part of the Y chromosome were demonstrable in both patients. The highly repetitive Y-specific DNA sequences located in the heterochromatic region of the long arm were absent in these patients. Differences in the composition of the euchromatic part of the Y chromosome were demonstrable in both patients. The suggestion that the Ynf chromosome originates from a dicentric Y chromosome cannot be accepted as a complete explanation of the phenomenon, as it probably involves more complex molecular alterations of the abnormal Y chromosome. The presence of Ynf is associated with the presence of a 45,X cell line more often than in cases of simple Y chromosome deletions with the breakpoint localized in or below the Y euchromatin/heterochromatin junction.