In girls, breast development before eight years of age is called “premature thelarche (PT)”. There are few studies in literature that show the interaction between PT and phthalate exposure. The aim of this study was to determine the urinary levels of di-(2-ethylhexyl) phthalate (DEHP) metabolites and other phthalate metabolites in girls with PT. PT group consisted of 29 newly diagnosed subjects. Control group comprised of healthy age-matched girls (n = 25). Urinary phthalate metabolite concentrations were measured by liquid chromatography/tandem mass spectroscopy (LC–MS/MS). The urinary concentrations of mono-(2-ethyl-hexyl)phthalate (MEHP) in the PT group (33.96 ± 6.88 μg/g creatinine) were found to be significantly higher compared to control group (11.54 ± 1.39 μg/g creatinine, p = 0.002). In PT group, %MEHP was also markedly higher vs. control (17.84 ± 3.31 vs. 6.44 ± 1.13, p = 0.001). Our results suggest that DEHP is more efficiently converted to MEHP in girls with PT, the importance of which needs to be further elucidated.