An object of the present invention is to provide pharmaceutical agents thatreducerisk for decrease in bone mineral density due to their effect of reducingestrogen levels andexert excellent therapeutic effects on endometriosis. The present inventionrelates topharmaceutical compositions for treating endometriosis comprising3-[2-fluoro-5-(2,3-difluoro-6-methoxybenzyloxy)-4-methoxyphenyl]-2,4-dioxo-1,2,3,4-tetrahydrothieno[3,4-d]pyrimidine-5-carboxylic acid or a pharmaceutically acceptable saltthereof, which areadministered orally once a day at a daily dose of between 50 mg and 75 mgcalculated as afree form.
