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Background: Early onset of breast development in a young girl is usually a benign and isolated prepubertal condition, i.e., premature thelarche (PT), but can sometimes be progressive and the first sign of pubertal precocity (PP). Serum 17β-estradiol (17β-E2) level is a possible marker to differentiate between benign and pathological forms of breast development. We defined an upper serum 17β-E2 level for benign, “classic” PT for girls aged 9–48 months. Methods: Serum 17β-E2 was analysed with a highly sensitive extraction radioimmunoassay (RIA). Gonadotropins, Tanner breast stage, growth, other investigations, and clinical outcome were assessed in 125 girls with breast development, in a population-based study in West Sweden. Results: A total of 125 of 128 girls had a benign form of breast development with a mean serum 17β-E2 level of 15.2 pmol/L and a mean + 2 SD of 31 pmol/L, which was regarded as the upper limit for benign PT; 3 girls with PP had 17β-E2 levels above 70 pmol/L. Conclusion: This is the first study to define an upper serum 17β-E2 level associated with benign PT. Girls aged 9–48 months with PT and Tanner breast stage 2 have 17β-E2 levels below 32 pmol/L using extraction RIA. LH below the detection limit (0.1 IU/L) and measurable FSH support benign PT. [ABSTRACT FROM AUTHOR]