Clinical features, urocytogram and gonadotropin response to LHRH (100 microgram/1,73 m2 i.v.) were evaluated in 10 girls with premature thelarche and 4 girls with precocious puberty to purpose to describe the differences between the two diseases. The subjects with Premature Thelarche showed peculiar clinical and endocrinological characteristics which allowed their discrimination from patients with Precocious Puberty: 1) breast development of mild degree without any modification of nipples and areolae and not accompanied by pubic hair or morphologic changes of genitalia; 2) spontaneous regression of thelarche more or less complete (6/10 cases); 3) not anticipation of puberal maturation; 4) growth rate not accelerate (only a mild advancement of statural age was observed); 5) LH/FSH ratio at prepuberal levels; 6) mean LH peak values overlapping to those obtained in normal infants under 1 year of age. Our findings confirm that: 1) the girls with Premature Thelarche showed different growth pattern and clinical features than those observed in the girls with Precocious Puberty; 2) LHRH test and urocytogram are further valuable parameters for the differential diagnosis between Premature Thelarche and Precocious Puberty; 3) the isolated breast enlargement in girls under 8 years of age is likely to originate from a protracted gonadotropin hyperstimulation as in early life; 4) it is a variant of the normal, not difficult to asses, which merits, however further follow-up.