Loss of SMARCB1/INI1 protein expression is considered useful for confirming a histologic diagnosis of malignant rhabdoid tumor. However, loss of SMARCB1/INI1 protein expression has recently been reported in other tumors as well, including a few cases of epithelioid sarcoma. In addition, the histopathologic differences between proximal-type epithelioid sarcoma and malignant rhabdoid tumor have not been conclusively defined. We analyzed SMARCB1/INI1 protein expression in 54 epithelioid sarcoma (proximal-type, 25; distal-type, 29) and examined alterations of the SMARCB1/INI1 gene in the cases lacking protein expression. We found that 19 (76.0%) proximal-type epithelioid sarcoma and 27 (93.1%) distal-type epithelioid sarcoma showed loss of SMARCB1/INI1 protein expression. Analysis of 39 cases with loss of protein expression revealed 4 cases (10.3%) with SMARCB1/INI1 gene alterations at the DNA level (homozygous deletion, 2; 1- or 2-bp deletion, 2) that could have induced the loss of gene products, and all 4 of these were proximal-type epithelioid sarcoma. Epithelioid sarcoma was thus associated with a high frequency of loss of SMARCB1/INI1 protein expression similar to that in malignant rhabdoid tumor. However, the frequency of SMARCB1/INI1 gene alteration at the DNA level in proximal-type epithelioid sarcoma was significantly lower than that in malignant rhabdoid tumor. In addition, the prognosis of patients with malignant rhabdoid tumor is significantly worse than that of patients with proximal-type epithelioid sarcoma (P = .001). Therefore, proximal-type epithelioid sarcoma and malignant rhabdoid tumor are suggested to be distinctive tumors with respect to the mechanism of the loss of SMARCB1/INI1 protein expression. Analysis of alterations in the SMARCB1/INI1 gene may thus be a useful diagnostic tool to distinguish proximal-type epithelioid sarcoma from malignant rhabdoid tumor.