Objectives: To determine whether age at premature thelarche (PT) onset affects the clinical characteristics, course, and risk of progression to precocious puberty (PP). Study design: Data regarding course of growth and puberty were retrieved from the medical files of 139 girls with PT followed up from 1995 to 2005. Analysis was based on age at PT appearance (birth, 1-24 months, and 2-8 years); course was categorized as regressive, persistent, progressive, or cyclic. Results: At diagnosis, height standard deviation score, bone age-chronological age ratio, and hormonal values were comparable in the 3 age groups. PT regressed in 50.8%, persisted in 36.3%, progressed in 3.2% and had a cyclic course in 9.7%. A progressive or cyclic course was significantly more prevalent among girls presenting after 2 years (52.6%) compared with girls presenting at birth (13.0%) or at 1 to 24 months (3.8%) (P < .001). PP occurred in 13% irrespective of age at PT presentation or clinical course. Conclusions: Clinical and anthropometric characteristics at admission and risk of PP were similar in all girls with PT, regardless of age at onset. There are currently no clinical or laboratory tests that can predict the risk of progression to PP at presentation.