Objective: To investigate the value of quantitative assay of urinary FSH and LH in differentiating classic from transitional premature thelarche. Methods: A total of 112 girls with breast development before 8 years of age and undergone both LHRH stimulation and urinary LH and urinary UFSH assay were enrolled. Of them 53 girls with CPP (central precocious puberty) started treatment immediately, whereas the other 49 girls were observed 6 months for pubertal advancement, gonadal hormone, height acceleration, and bone age maturation, and were categorized into two groups: the classic premature thelarche group and the transitional premature thelarche group. Results: In both groups, the serum LH level was positively correlated with morning urinary LH level. The morning urinary LH/FSH of girls at first visit in classic premature thelarche and transitional premature thelarche were 0.50±0.35 and 1.16±1.08, respectively, the difference was statistically significant (P<0.05). Urinary LH/FSH could differentiate girls with classic premature thelarche from transitional premature thelarche. Receiver operating characteristics analysis showed an LH/FSH cutoff value of 0.512, sensitivity of 80% and specificity of 56%. Conclusions: Urinary LH/FSH assay is a noninvasive, reliable method that can assist the differentiation between classic from transitional premature thelarche.