Mucosa-associated lymphoid tissue (MALT) lymphoma is a relatively common, indolent B-cell lymphoma. MALT lymphoma with large tumor cells (LTCs) is believed to have the potential to transform to aggressive diffuse large B-cell lymphoma (DLBCL) which may have a poor prognosis. C-MYC is a transcription factor. Its translocation and overexpression predicts an inferior prognosis and poor response to therapy in cases of DLBCL. In the current study, C-MYC expression was detected in MALT lymphomas, and its relationship to the occurrence of LTCs, clinicopathological parameters and prognosis was assessed. A total of 69 cases were enrolled in the study, including 42 cases of MALT lymphoma without LTCs, 20 cases of MALT lymphoma with LTCs and 7 cases of DLBCL with a MALT lymphoma component (DLBCL+MALT). Immunohistochemistry and fluorescent in situ hybridization analyses were performed. In total, 15/42 (35.7%) cases were nuclear positive for C-MYC expression in the group without LTCs, whereas 15/20 (75.0%) and 4/7 (57.1%) cases were positive in the group with LTCs and in the group with DLBCL+MALT, respectively (P=0.004). Univariate and multivariate analysis were used to determine the correlations of C-MYC expression and clinicopathological parameters with overall survival (OS). C-MYC expression, Ann Arbor stage, LDH level and IPI were considerably associated with OS according to the univariate analysis. However, only C-MYC expression ≥ 20% showed a statistical significance in the multivariate analysis (HR=20.604, 95% CI: 1.909-222.412, P=0.013). Therefore, C-MYC overexpression may play an important role in aggressive transformation and is an independent prognostic factor in MALT lymphoma.