Early diagnosis of cancer offers the best chance for effective treatment. Serological tests for the detection of cancer testis antigens (CTA) could aid in cancer diagnosis, prognosis, and treatment evaluation. Since NY-ESO-1 induces a strong immune response in several cancer types, it is considered an attractive CTA for antibody assay development. The full-length NY-ESO-1 protein was recombinantly produced in Escherichia coli, purified, and used to develop an indirect enzyme-linked immunosorbent assay (ELISA) for the detection of NY-ESO-1 specific antibodies. This ELISA was optimized and validated using serum samples collected from 78 confirmed cancer patients and control sera from healthy donors (n = 20) and rheumatoid arthritis patients (n = 43). The optimized NY-ESO-1 ELISA provided a sensitivity of 96.2 % in cancer patients and a specificity of 100 % for non-cancer donors. A follow-up study of patients undergoing cancer treatment indicated that the antibody levels were responsive to therapy, becoming very low or negative after successful treatment and rising again before tumor metastasis recurrence was confirmed. Comparative peptide ELISA using overlapping peptides of the full NY-ESO-1 sequence did not improve the detection rate, but did reveal common epitopes. The NY-ESO-1 protein-based ELISA could be used to detect potential cancer patients and is useful for monitoring cancer therapy and tumor recurrence after treatment.