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Objective: To determine whether decreased testicular levels of enzymes necessary for retinoic acid biosynthesis were associated with male infertility, as retinoic acid is known to be necessary for spermatogenesis. Design: Observational analysis of testicular tissue samples, sperm indices, and serum hormone concentrations. Setting: Two infertility centers in Chile. Patient(s): 32 infertile men and 11 control men. Intervention(s): Measurement of the three enzymes necessary for retinoic acid biosynthesis, aldehyde dehydrogenase (ALDH) 1A1, 1A2, and 1A3, in testicular tissue by a novel liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) peptide assay. Main Outcome Measure(s): ALDH isozyme levels compared by type of infertility and correlated with testicular germ cell numbers, sperm parameters, and serum and intratesticular hormone concentrations. Result(s): Men with infertility had statistically significantly reduced levels of ALDH1A2 but not ALDH1A1 or ALDH1A3 in their testicular tissue compared with men with normal spermatogenesis. The ALDH1A2 protein levels were strongly correlated with the number of germ cells found via testicular biopsy. Conclusion(s): These findings suggest that ALDH1A2 is the enzyme involved in retinoic acid biosynthesis in human germ cells. Further study of the relationship between intratesticular ALDH1A2 and male infertility is warranted to determine whether men with infertility have a reduced ability to synthesize retinoic acid within their germ cells that could impair spermatogenesis. [ABSTRACT FROM AUTHOR]