Purpose: We evaluated the clinical characteristics of men with uniform testicular maturation arrest and nonobstructive azoospermia or severe oligospermia, including the frequency of genetic defects and outcome of intracytoplasmic sperm injection with or without testicular sperm extraction.Materials and Methods: We identified a group of 32 men with nonobstructive azoospermia or severe oligospermia, uniform maturation arrest (single spermatogenic pattern on biopsy), and normal follicle-stimulating hormone (7.6 IU/l or less). These patients were identified from 150 intracytoplasmic sperm injection candidates with severe oligospermia (less than 10,000/cc) and 600 men with nonobstructive azoospermia undergoing attempted testicular sperm extraction-intracytoplasmic sperm injection between November 1995 and September 2006. These patients were characterized based on the frequency of genetic anomalies (karyotype or Y chromosome microdeletions). Rates of sperm retrieval by testicular sperm extraction, fertilization...