An approach to the sonographic differential diagnosis of pelvic masses based on their size, location, internal consistency, and definition of borders is presented. Diagnostic schemes were derived from correlating the sonographic features with histomorphology in 170 surgically proven pelvic masses. Besides separating pelvic masses into the conventional categories of cystic, complex, and solid, gray scale sonographic features of a pelvic mass can be used to subcategorize these masses into a more useful differential diagnoses. Although the features seen on a sonographic image were specific in only two types of pelvic masses (pattern specificity greater than 85%), the sonographic information can be effectively utilized for establishing differential diagnoses of pelvic masses. Among the various sonographic patterns observed, homogenously cystic adnexal masses were the least specific pattern, whereas a predominantly cystic, extrauterine mass with internal septation was highly specific for a pseudomucinous cystadenoma. Several pelvic masses such as dermoid cysts demonstrated more than one sonographic appearance and, therefore, had to be considered in more than one diagnostic category.