Premature thelarche (PT) is characterised by precocious breast development without any other sign of puberty, normal height velocity (HV) and normal bone maturation, while girls with central precocious puberty (CPP) show increased HV, bone maturation and increased serum IGF-I and IGFBP-3 levels. This prompted us to study serum IGF-I and IGFBP-3 concentrations in girls with PT. Thirty-nine girls with premature breast development were studied and classified as PT or CPP according to clinical and laboratory evaluation. Normal prepubertal and pubertal girls were studied as controls. Serum IGF-I and IGFBP-3 were determined in all girls by IRMA. IGF-I levels in PT (155 +/- 61 microg/l) were lower than in CPP (337 +/- 149 microg/l) or late-pubertal controls (355 +/- 84 microg/l) and similar to those found in prepubertal (113 +/- 72 microg/l) and early-pubertal (222 +/- 81 microg/l) girls. Considering the SDS of IGF-I for chronological age (CA), the values observed in PT were in an intermediate position between CPP and prepubertal controls and statistically similar to those observed in CPP and prepubertal girls. IGFBP-3 levels in PT (2.1 +/- 0.5 mg/l) were similar to those found in CPP (2.5 +/- 0.8 mg/l), but only the latter were higher than in prepubertal girls (1.9 +/- 0.9 mg/l). IGF-I/IGFBP-3 molar ratios in PT were in an intermediate position between CPP and prepubertal controls. In conclusion, IGF-I and IGF-I/IGFBP-3 values in PT are intermediate between those observed in prepubertal children and in CPP, suggesting that PT could be a very early stage of puberty with slight but real changes in the GH-IGF axis.