The clinical utility of transvaginal ultrasonography in the differentiation of adenomyoma from leiomyoma was evaluated in 147 patients who had been scheduled for surgery due to symptomatic uterine masses. In all subjects, ultrasonographic images obtained preoperatively were correlated postoperatively with surgicopathological findings. Pathological findings showed that 110 patients proved to have fibroids, while 30 had adenomyomata. For the diagnosis of adenomyoma, transvaginal ultrasonography attained a sensitivity of 80%, a specificity of 94.3%, a positive predictive value of 85.7% and a negative predictive value of 90.9%, compared with a sensitivity of 94.3%, a specificity of 80%, a positive predictive value of 90.9% and a negative predictive value of 85.7% for leiomyoma diagnosis. Further to assess which characteristic used in ultrasonography was useful in the differential diagnosis, five characteristics were analyzed and compared by chi 2 test. These were position, number, margin and echogenicity of the uterine masses and the presence or absence of hypoechoic spaces (lacunae). Margin, echogenicity, mass number and lacunae were significantly different between both conditions. A stepwise logistic regression procedure revealed that margin, lacunae and echogenicity were good parameters for differentiating adenomyoma from leiomyoma. If we selected the features of distinct margin and absence of hypoechoic lacunae within the masses for analysis, leiomyoma could be correctly predicted in 97% of patients.