Low-grade serous carcinoma is a relatively recent recognized entity. The cytologic features of this type of carcinoma have been described in detail; however, the different histologic patterns of this carcinoma have not been discussed yet. We reviewed the hematoxylin and eosin-stained slides and studied the different patterns of invasion and the histologic features of areas that were not obviously invasive of 40 cases of ovarian pure low-grade serous carcinoma. Destructive areas of invasion were present in all cases; however, in 25 of the 40 cases obvious invasive areas were mixed with foci that were difficult to recognize as invasive. The obvious areas of invasion were characterized by the presence of tumor cells within nonepithelial-lined spaces, which are clefts in the stroma surrounding the tumor cells. The different patterns of the tumor cells within the nonepithelial-lined spaces and the frequency that they are found were: groups or nests (nidi pattern) in 100% of the cases, micropapillae in 70%, these 2 patterns frequently were mixed, macropapillae in 25%, large solid groups in 5%, and single cells in 3%. The 2 patterns difficult to recognize as invasive because of the absence of nonepithelial-lined spaces were disorganized orphan papillae within epithelial-lined spaces present in 40% of the cases, and small irregular glands with a haphazard distribution infiltrating the tissue eliciting desmoplastic stromal reaction present in 22% of the cases. Calcifications were present in all cases, and mucin in 70% of the cases. Areas of stromal invasion characterized by tumor cells within nonepithelial-lined spaces or clefts are seen in all low-grade carcinomas. Within these spaces, small groups of tumor cells are found frequently mixed with micropapillae. In 62% of the cases, in addition to obvious areas of invasion, there were 2 patterns difficult to recognize as invasive, disorganized orphan papillae, and irregular well-developed glands.