The aim of this study was to determine reliable cut-off levels of basal gonadotropin and to assess the correlation of obstetric ultrasound parameters with the GnRH stimulation test. The GnRH stimulation test was performed in a cohort of young female patients who presented at our hospital for the evaluation of early signs of puberty. Using receiver operating curves (ROCs), the sensitivity and specificity of basal luteinising hormone (LH), follicle stimulating hormone (FSH), basal and stimulated LH/FSH ratio, oestradiol levels and ultrasonographic parameters were evaluated at each level, and the area under curve (AUC) was measured. One hundred female children were assessed. We found that LH levels, peak LH/FSH ratio, fundal/cervical ratio, uterus length, and ovarian volume were reliable predictors of central precocious puberty (CPP). Cut-off levels of basal LH and the peak LH/FSH ratio had high specificity in our cohort. In addition, obstetric ultrasound parameters represent reliable predictors for the diagnosis of CPP.