A positive correlation between the severity of endometriosis and the percentage of CD4+CTLA-4+ T cells (Spearman’s R = 0.531, p < 0.001), and a positive correlation between the severity of endometriosis and the percentage of CD8+CTLA-4+ T cells (Spearman’s R = 0.450, p = 0.001) were observed. The gene discussed is CTLA4; the disease is endometriosis.