Background: Appendiceal cancers are rare tumors with prognostic heterogeneity and treatment-responses dependent upon histologic characteristics. In the 7^th edition AJCC manual, mucinous tumors were separated from non-mucinous. Histologic grade was used to distinguish stage IVa from IVb in mucinous tumors. We examined SEER data to investigate the impact of cancer grade on all stages of appendiceal cancer. Methods: We identified patients (pts) with primary appendiceal cancer from the SEER data. Disease-specific survival (DSS) was analyzed based on histologic subtype, stage, and grade. Hazards ratios were calculated using Cox models. Tumor grades were grouped according to AJCC criteria (Grade 1 vs. Grade 2 and above). Results: From 1988-2011 a total of 4491 appendiceal adenocarcinomas were identified in the SEER database; 2026 (45%) pts had mucinous histology and 1578 (35%) had non-mucinous histology. Tumor grade had no impact on DSS in stage I and II tumors. However, in Stage III and IV mucinous tumors higher tumor grade was significantly correlated with worse survival. In pts with Stage III mucinous disease, those with grade 1 tumors had significantly longer DSS than those with tumors ≥ grade 2, with 5-year survival of 75% vs. 44%, respectively (p = 0.02, HR 3.15, 95% CI 1.11 - 8.96). Survival for mucinous stage III grade 1 tumors was similar to stage II tumors. Conclusions: Tumor grade is a strong prognostic indicator in pts with stage III mucinous appendiceal carcinoma. We propose that tumor grade should be included in AJCC staging for stage III mucinous appendiceal cancers, and should not be limited to Stage IV disease as in the AJCC 7^th staging manual. Continued investigation into the clinical implications of the observed difference in survival in patients with Stage III appendiceal mucinous adenocarcinoma is warranted, and may potentially alter adjuvant treatment recommendations in selected pts.