Context: Despite the importance of dihydrotestosterone in androgen action, polymorphisms in the genes for the two isoforms of 5α-reductase (SRD5A1 and SRD5A2) have not been evaluated as risk factors for polycystic ovary syndrome (PCOS). Objective: The objective of the study was to test the hypothesis that haplotypes in the SRD5A1 and SRD5A2 genes are risk factors for PCOS and the severity of hirsutism in affected women. Design: PCOS and control subjects were genotyped for seven singlenucleotide polymorphisms in SRD5A1 and eight single-nucleotide polymorphisms in SRD5A2. Haplotypes were determined and tested for association with PCOS diagnosis and component phenotypes. Setting: Subjects were recruited from the reproductive endocrinology clinic at the University of Alabama at Birmingham; control subjects were recruited from the general surrounding community. Genotyping took place at Cedars-Sinai Medical Center in Los Angeles. Participants: A total of 287 White women with PCOS and 187 controls participated. Main Measurements: SRD5A1 and SRD5A2 genotype, quantitative hirsutism score, and hormonal and metabolic phenotypes were assessed. Results: Haplotypes within both genes were associated with PCOS risk. The Leu allele of the Val89Leu variant in SRD5A2 was associated with protection against PCOS; this allele is known to modestly reduce 5α-reductase activity. Haplotypes in SRD5A1 but not SRD5A2 were also associated with the degree of hirsutism in affected women. Conclusions: This study presents genetic evidence suggesting an important role of both isoforms of 5α-reductase in the pathogenesis of PCOS. That only SRD5A1 haplotypes were associated with hirsutism suggests that only this isoform is important in the hair follicle. Copyright © 2006 by The Endocrine Society.