Objective: To determine whether meiotic and postmeiotic germ cell gene products could be detected in biopsy specimen from patients with Sertoli cell only (SCO) and maturation arrest.Design: Prospective clinical study.Setting: University-based departments and laboratories.Patient(s): Nine patients, seven with nonobstructive azoospermia (12 biopsies) and two with obstructive azoospermia (controls) (2 biopsies).Intervention(s): Specimens were divided into three parts: IVF laboratory, histology, and molecular analysis. Germ cell-specific messenger RNAs (mRNAs) were detected by extracting total RNA for Northern blotting or reverse transcription-polymerase chain reaction.Main Outcome Measure(s): Detection of meiotic (lactate dehydrogenase C4) and postmeiotic (transition protein 1 and protamine 1 and 2) gene expression and correlation with histologic and IVF laboratory findings.Result(s): The IVF laboratory identified spermatozoa in 3 of 14 biopsies (controls and severe hypospermatogenesis). Histologically, 6 of 14 biopsies (43%) were diagnosed as SCO, 4 (29%) maturation arrest, 2 (14%) severe hypospermatogenesis, and 2 normal. Molecular analysis showed mRNA for meiotic and postmeiotic genes in 12 of 14 biopsies (86%) (P = .006), of which 4 (67%) in SCO and 3 (75%) in maturation arrest.Conclusion(s): Differentiated germ cells are present in biopsies of men histologically diagnosed as SCO. Absence of these molecular markers strengthens the histologic diagnosis and helps the physician in counseling the infertile couple.