Objectives: Microdeletions of the long arm of Y chromosome, particularly interval 6, are associated with severe spermatogenic impairments. Generally, microdeletions have occured in at least one of three regions, designated azoospermic factor (AZF) a, AZFb and AZFc (Vogt et al., 1996). Gene in AZFc region was identified to deleted in azoospermia(DAZ), but gene in AZFa, b region were not identified yet. The objectives of this study were to evaluate the incidence of microdeletions and to determine the frequency of AZFa, b, c deletions in idiopathic, obstructive, nonobstructive azoospermia, severe oligoasthenoteratozoospermia (OATS) and in normal males from Korean. And we analyzed the clinical characteristics including histopathology and volume of testis, and hormone value in serum from microdeleted patients.Designs: We investigated microdeletions of the Y chromosome from 115 nonobstructive, 30 obstructive azoospermia, 23 severe OATS patients, and 43 normal males in Korean men.Materials and Methods: Blood samples were taken to detect microdeletions of the AZF region by polymerase chain reaction (PCR). We tested the presence of sequence taged site (STS) markers(DAZ, sY129, sY134, sY84 and SRY), which are known to be prevalent in Y chromosome microdeletions. PCR results were confirmed by Southern hybridization, and PCR products were analysed by single strand conformational polymorphism for DAZ gene mutation.Results: None of 43 normal males and 30 obstructive azoospermia had microdeletions. 3 patients (13.04%) had microdeletion of one or more STS markers in the 23 severe OATS, and 14 patient (12.17%) had microdeletion of one or more STS markers in the 115 nonobstructive azoospermia. The microdeletion frequency of AZFa, b, c regions in non-obstructive azoospermia and severe OATS patients were 0.72%(1138), 5.80%(8138), 8.70%(12138) respectively. Testicular pathology were Sertoli cell only, maturation arrest and hypospermatogenesis in microdeleted patients. And testicle volume and hormone value in serum were resulted in broad range from normal to abnormal in microdeleted patients.Conclusions: From these results, high prevalence (12.32%) of Y chromosome microdeletions (DAZ, sY129, sY134 and sY84) in men with nonobstructive idiopathic azoospermia and severe OATS were observed. Also the microdeletions were more prevalent in AZF b, c regions than AZFa region in Korean infertility patients (P<0.05). Therefore, analysis of AZF deletions will be needed in diagnosis of male infertility. And preimplantation genetic diagnosis will be needed in assisted reproductive technique program to avoid the infertile offspring by intracytoplasmic sperm injection or round spermatid injection.