The occurrence of regional lymph node involvement (LNI) in patients with primary ovarian serous tumors of low malignant potential (S-LMP), although well described in the literature, continues to be problematic. Most studies indicate that LNI is not associated with an adverse prognosis, but there has not been a comprehensive study addressing the histologic patterns of LNI, the importance, if any, of classifying the type of LNI (ie, as either noninvasive or invasive in analogy to peritoneal implant classification), or the presence and significance of associated endosalpingiosis. To further evaluate LNI in S-LMP, 74 patients with ovarian S-LMP and a lymph node biopsy or sampling were studied. Thirty-one of 74 patients had LNI in pelvic (18; 58%), mesenteric/omental (9; 29%), paraaortic (8; 26%), or supradiaphragmatic (2; 6%) lymph nodes. The number of involved nodes ranged from 1 to 20 (mean, 11.1). Four patterns of LNI were identified: individual cells, clusters of cells, and simple, nonbranching papillae (28 of 31; 90%); intraglandular (21 of 31; 68%); cells with prominent cytoplasmic eosinophilia (“eosinophilic cell” pattern) (16 of 31; 52%); and micropapillary pattern (5 of 31; 16%). LNI was diffuse in at least one lymph node in 13 patients (42%) and formed nodular aggregates greater than 1 mm in 6 patients (19%). Nodal endosalpingiosis was present in 58% of cases with LNI compared with 35% without LNI (P=0.06). There was no significant difference in survival for patients with LNI compared with patients without LNI. However, the presence of discrete nodular aggregates of epithelium greater than 1 mm in linear dimension without intervening lymphoid tissue was associated with a statistically significant decreased disease-free survival when compared with other patterns of LNI (P=0.02). Nodular aggregates were strongly associated with desmoplastic fibrous stromal reaction (P=0.001) and micropapillary architecture (0.02). There was also a trend for decreased survival among patients with LNI without associated endosalpingiosis (56%) compared with patients with LNI associated with endosalpingiosis (85%) and those with endosalpingiosis only (93%). This study suggests that patients with ovarian S-LMP may be further substratified into risk categories by the presence of nodular aggregates of S-LMP in lymph nodes, a feature that is more common in cases with micropapillary architecture and associated stromal reaction in the intranodal tumor. This high risk pattern of LNI may have a predictive value similar to invasive peritoneal implants and deserves independent evaluation in future studies of S-LMP.