NY-ESO-1 antigen is a prototype of a class of cancer/testis antigens. We carried out three serial clinical trials using NY-ESO-1 whole protein, NY-ESO-1f long peptide and overlapping peptides as a cancer vaccine for advanced cancer patients. Although vaccines elicited NY-ESO-1 humoral and cellular immune responses in most patients, results of immune monitoring using peripheral blood was not consistent with clinical responses. Analysis of immunohistochemistry revealed possible involvement of regulatory mechanisms in local antitumor immunity. Immune checkpoint molecules, e.g., CTLA-4 and PD-1, on immune effector cells, and regulatory T cells are known to regulate the induced immune responses and minimize local tissue damage. We should take these factors into consideration for further strategy of cancer vaccine and its monitoring.