We have detected deletions of a Y-specific microsatellite marker, Yfm1, located on the Y chromosome (Yq) within interval 6 and near the DAZ (deleted in azoospermia) genes, in 9/89 oligospermic and 17/68 azoospermic Japanese men. No Yfm1 deletions were detected in the 150 normal fertile males examined as controls. Yfm1 deletions in the oligo- and azoospermic males were associated with other deletions that removed entire DAZ genes in those infertile men. These deletions indicated that all Yfm1 loci are located within azoospermia factor c (AZFc) in interval 6 on the long arm of the Y chromosome. Mapping Yfm1 on the Y chromosome using the draft sequence of the human genome revealed that at least three Yfm1 loci are located within about 25-30 kbp of the DAZ genes. Moreover, the Yfm1 marker showed the least number of copies in Japanese males derived from a Y chromosomal lineage called haplotype II, defined by having the Y Alu polymorphism (YAP) insertion. Males from this haplotype II lineage are known from our previous studies to have lower spermatogenic abilities, with higher rates of oligo- and azoospermia than other haplotypes. The least number of Yfm1 loci, whose copy number may correspond to that of the DAZ genes, may be a risk factor predisposing an individual to azoospermia or oligospermia.