Phthalates may act as an estrogen and are a potential risk factor for estrogen-related diseases such as endometriosis. We assessed the association between phthalate exposure and endometriosis in 166 consecutive women who presented at a university hospital for consultation regarding infertility. The subjects were interviewed and provided a urine specimen prior to a laparoscopic diagnosis of endometriosis. They were then categorized by the severity of endometriosis as controls (stages 0-I) and cases (stages II-IV). Urinary concentrations of the phthalate metabolites monoethyl phthalate, mono-n-butyl phthalate, monobenzyl phthalate, mono(2-ethylhexyl) phthalate (MEHP), mono(2-ethyl-5-oxohexyl) phthalate, and mono(2-ethyl-5-hydroxyhexyl) phthalate were measured in 57 cases and 80 controls using high-performance liquid chromatography isotope-dilution tandem mass spectrometry. Adjusted odds ratios for endometriosis in relation to dichotomized individual phthalate metabolites (standardized for creatinine) were calculated. No significant association between endometriosis and any urinary creatinine-adjusted phthalate monoester was seen. Adjusted odds ratio (95% confidence interval) for higher dichotomized MEHP by endometriosis was 1.57 (0.74-3.30). No monotonic trend was seen in urinary creatinine-adjusted concentration of phthalate metabolites by endometriosis stage (p=0.23-0.90). Our results do not support the hypothesis that higher urinary concentrations of phthalate metabolites are associated with the risk of endometriosis in infertile Japanese women.