Background: Endometrioid endometrial cancer is the most common histological subtype of endometrial adenocarcinoma. In the FIGO grading scheme, both architectural and nuclear grade are taken into consideration. However, the specific impact of solid growth alone on endometrioid endometrial adenocarcinoma outcome is not well documented. We sought to assess the degree of impact of solid growth on lymphovascular space invasion (LVSI), myometrial invasion, tumor size, FIGO stage, lymph node metastasis (LNM), relapse-free survival (RFS) and disease-specific survival (DSS). Methods: Paraffin blocks of 269 patients treated for endometrioid endometrial cancer were retrospectively analyzed with morphometry for solid growth percentages. Results: A statistically significant cut-off value of 1% solid growth was found for predicting LNM and advanced stage (III or IV), myometrial invasion and LVSI (p < 0.001) and a cut-off value of 8% was found for predicting adverse survival outcome (p < 0.001). The mean DSS was significantly higher in patients with < 6% solid growth compared to patients with 6–50%, 51–75% and > 75% solid growth (p < 0.001). Although, the mean RFS and DSS were lowest in patients with 51–75% solid growth, this did not reach statistical significance in comparison to 6–50% and > 75% (p > 0.05). Conclusion: Although > 75% solid growth was most significantly associated with many of the adverse prognostic factors, this subset did not provide prognostic superiority in predicting adverse survival when compared to subsets within 6–75% solid growth. In conclusion, although no statistically significant difference in survival was found among subdivisions of architectural grades 2 and 3, solid growth, especially ≥ 8%, appeared to be an independent prognostic factor for survival in patients with early-stage endometrioid endometrial cancer.