The real-time pelvic sonograms of 32 girls under 8 years old with true isosexual precocity (23 cases), pseudosexual precocity (four cases), premature adrenarche (four cases), and an undetermined problem (one case) were evaluated retrospectively for ovarian volume and presence and size of cysts. Sonographic findings were compared to those of 181 age-matched controls to determine the best sonographic indicator of precocious puberty. Ovarian volume was 4.6 cm3 in girls with true isosexual precocity, 4.1 cm3 in girls with pseudosexual precocity, and less than 1 cm3 in the other patients as well as in the control population. Ovarian enlargement was bilateral in true precocity and unilateral in pseudosexual precocity. Of the 181 subjects in the control group, 96 (53%) had ovarian cysts, almost all of which were small (less than 9 mm in diameter). Ovarian cysts occurred in 22 of 32 patients (69%) with precocity. The cysts generally were smaller than 9 mm in true isosexual precocity and larger than 9 mm in pseudosexual precocity. In conclusion, small ovarian cysts are not specific to precocious puberty and its various subtypes. Bilateral ovarian enlargement appears to be a reliable indicator of true isosexual puberty, whereas unilateral ovarian enlargement in combination with macrocysts is suggestive of pseudosexual precocity.