Experience with endometrial stromal nodules (ESNs) is limited, and no series has been published since the frequent misdiagnosis of highly cellular leiomyomas as ESNs was highlighted. Additionally, although the entirely well-circumscribed margin of most ESNs readily allows a separation from endometrial stromal sarcomas with their typical permeative invasion, some tumors have greater irregularity of their margin than allowable for an ESN following the guidelines of Tavassoli and Norris, but without the typical, and usually extensive, infiltration of endometrial stromal sarcomas. These have been diagnosed by us descriptively as endometrial stromal tumors with limited infiltration. Three cases of this type and 47 ESNs that occurred in patients from 31 to 86 years (mean 53 years) were analyzed. The tumors were typically fleshy and at least in part yellow and ranged from 1.2 to 22 cm (mean 7.1 cm). Seven tumors had one to six focal margin irregularities; in four tumors these did not extend >3 mm beyond the main border of the tumor and were up to three in number. The other three (endometrial stromal tumors with limited infiltration) had four to six irregularities extending up to 9 mm beyond the main border of the tumor. Most neoplasms were densely cellular, but nine were hypocellular with areas that were fibrous, hyalinized, myxoid, edematous, or combinations thereof. The tumor cells almost always had scant cytoplasm, but in one tumor many cells had abundant eosinophilic cytoplasm. There was minimal cytologic atypia. Forty-four tumors had mitotic rates of up to 5 per 10 high power fields, but six had higher rates. Typical arterioles, some of which had hyalinized walls in nine cases, were numerous in all 50 tumors but were conspicuous on low-power examination in only 22. Larger, sometimes thick-walled vessels were seen in 37 tumors but were much less conspicuous than in highly cellular leiomyomas and were often present near the margin of the tumor. The cleft-like spaces of highly cellular leiomyomas were not a feature of these tumors. Twenty-three tumors showed variable degrees of smooth muscle differentiation and sex cord-like differentiation occurred in 12. Cysts were present in 17 tumors and infarct-type necrosis in 34, the latter sometimes causing initial concern for a malignant neoplasm. Follow-up information, available for 32 patients, including five patients whose tumor had some margin irregularity, ranged up to 214 months (mean 43.5 months). All patients were alive with no recurrence. Follow-up information was unfortunately not available for the two tumors with the greatest degree of infiltration. The major conclusions of our study are as follows: ESNs are characteristically soft and yellow; thorough sampling of their margin is mandatory; smooth muscle metaplasia is common and irregular interdigitation of stromal neoplasia with metaplastic smooth muscle may erroneously suggest myometrial infiltration; although typically densely cellular about 20% of ESNs are hypocellular and often fibrous; cellular neoplasms are often punctuated by hyaline plaques; sex cord-like differentiation, broad zones of necrosis and rarely epithelioid cells may complicate the microscopic appearance; typical arterioles are an important microscopic finding but are not always striking, particularly on low-power evaluation; the tumors may have a component of large blood vessels more typical of highly cellular leiomyomas, but they are not as conspicuous as in the latter and the clefts that are frequent in highly cellular leiomyomas are not a feature of ESNs.