Posterior urethral polyp is a rare disease, predominantly affecting boys. The authors report the case of an 10-month-old boy admitted to hospital with acute urine retention. The diagnosis of polyp was established on cystourethrography, ultrasonography and endoscopy. The polyp was resected via a bladder incision after failure of transurethral resection. Posterior urethral polyp is a benign tumour whose clinical presentation is dominated by signs of lower urinary tract obstruction. Cystourethrography and ultrasonography confirm the diagnosis and assess the repercussions on kidneys and bladder. Endoscopy has a double role: diagnostic and therapeutic.