The distinction between low-grade and high-grade disseminated appendiceal mucinous neoplasms is of critical importance in assessing prognosis and guiding patient therapy. SMAD4 encodes a protein that is a central component of the TGFβ signal transduction pathway, and loss of SMAD4 expression has been associated with poor prognosis in carcinomas of the gastrointestinal tract. We reviewed the clinicopathologic and molecular features of 109 disseminated appendiceal mucinous neoplasms identified over an 8-year period at our institution in an attempt to: (1) correlate SMAD4 immunohistochemical expression with tumor grade; and (2) assess the prognostic significance of SMAD4 expression in predicting overall survival. Compared with tumors demonstrating preserved SMAD4 expression, tumors with loss of SMAD4 expression more frequently exhibited high cytologic grade (85% vs. 50%, P=0.035), high cellularity (100% vs. 45%, P<0.001), and destructive invasion (100% vs. 55%, P=0.001). SMAD4 expression significantly correlated with overall tumor grade (P=0.003): all 13 tumors with loss of SMAD4 expression were high grade, whereas all 42 low-grade tumors displayed preserved SMAD4 expression. A significantly higher proportion of tumors with loss of SMAD4 immunohistochemical expression demonstrated loss of heterozygosity at chromosome 18q (38%) compared with tumors with preserved SMAD4 expression (11%) (P=0.049), suggesting that loss of SMAD4 expression is due to genomic deletion in a high proportion of cases. Patients with SMAD4-negative tumors had significantly worse overall survival in comparison with patients with preserved SMAD4 expression (log rank P=0.023). However, our multivariable analysis found that SMAD4 expression was not independent of overall tumor grade in predicting overall survival. Our results indicate that loss of SMAD4 immunohistochemical expression is associated with loss of heterozygosity at chromosome 18q and is always associated with aggressive histologic features in disseminated appendiceal mucinous neoplasms. SMAD4 immunohistochemistry may be a useful ancillary study in select cases of disseminated appendiceal neoplasia, in which the distinction between low-grade and high-grade tumors is difficult.