Objective: To conduct a prospective study of the potential prognostic role of endothelin-1 (ET-1) in a cohort of primary high-grade non-muscle-invasive urothelial bladder cancer patients, who were treated with adjuvant intravesical Bacillus Calmette-Guérin (BCG). Material and methods: Patients with primary high-grade nonmuscle- invasive urothelial bladder cancer, who received postoperatively induction and maintenance BCG therapy, were prospectively included. Recurrence and progression were histologically proven. Immunohistochemical staining for ET-1 was assessed. Epidemiological, pathological and clinical parameters as well as the expression of ET-1 in tumor specimens were statistically analyzed for recurrence, progression, recurrence-free survival (RFS) and progression-free survival (PFS). Results: ET-1 associates significantly with recurrence (p = 0.000), progression (p = 0.000), RFS (p = 0.000) and PFS (p = 0.000). The patient’s age is also significant for recurrence (p = 0.003, OR = 1.273 95% CI: 1.086-1.492) and RFS (p = 0.013). Conclusions: ET-1 seems to deteriorate prognosis in patients suffering from primary high-grade non-muscle-invasive urothelial bladder cancer, who are treated with adjuvant BCG instillations. Furthermore, the patient’s age associates with an increased likelihood for recurrence.