Objective: To investigate the possible role of phthalate, a ubiquitous chemical used in consumer products, in the pathogenesis of uterine leiomyoma. Design: Experimental and prospective case-control study using human samples. Setting: University hospital. Patient(s): Fifty-three women with histologic evidence of uterine leiomyoma and 33 surgical controls without leiomyoma. Intervention(s): Human myometrial and leiomyoma cells were treated with di-(2-thylhexyl)-phthalate (DEHP). Main Outcome Measure(s): Cell viability assay and Western blot analyses after in vitro DEHP treatment; high-performance liquid chromatography electrospray ionization tandem mass spectrometry in cases and controls. Result(s): In vitro treatment with DEHP led to an increased viability and increased expressions of proliferating cell nuclear antigen, B-cell lymphoma 2 protein, and type I collagen in myometrial and leiomyoma cells. The urinary concentration of mono-(2-ethyl-5-carboxypentyl) phthalate was higher in women with leiomyoma compared with controls. Conclusion(s): These findings suggest that exposure to phthalate may play a role in the pathogenesis of uterine leiomyoma by enhancing proliferative activity, exerting an antiapoptotic effect, and increasing collagen contents in myometrial and leiomyoma cells. (C) 2017 by American Society for Reproductive Medicine.