Objective: To determine whether the tyrosine autophosphorylation of the insulin receptor (IR) in the ovary of a woman with polycystic ovary syndrome (PCOS) was reduced compared to normal. Design: Experimental study.Setting: Tertiary care medical center. Patient(s): One woman with PCOS and one healthy control, both of whom underwent a hysterectomy and bilateral salpingo-oophorectomy. Intervention(s): Plasma membrane fraction of ovarian tissue was isolated, and the IR was purified and concentrated. Main Outcome Measure(s): Western blots of the IR, which had been incubated with and without insulin, were prepared. Colorimetric and chemiluminescent methods were used to detect the presence of the IR β-subunit and IR tyrosine autophosphorylation, respectively. Result(s): The presence of the β-subunit of the IR was identified in both ovarian samples. The degree of insulin-stimulated IR tyrosine autophosphorylation, reflected by the mean (±SD) relative optical density of the 95 kd band, was 4.3-fold higher in the normal ovary compared to the PCOS ovary (0.56 ± 0.18 optical density vs. 0.13 ± 0.10 optical density, respectively). Conclusion(s): Tyrosine autophosphorylation of the IR may be decreased in the ovaries of women with PCOS, similar to the findings in other tissues. The mechanisms through which insulin acts to produce an excess in ovarian androgen production in the face of a decrease in ovarian IR autophosphorylation remain to be determined. Copyright © 2001 American Society for Reproductive Medicine.