One of the basic abdominal organs that is assessed during transabdominal ultrasound examination for urological reasons is the urinary bladder. The bladder must be filled with urine. This is a prerequisite for a reliable assessment and, at the same time, an acoustic window in examining adjacent structures and organs, for instance the prostate gland. In some cases, doubts occur with respect to the nature of lesions detected. The paper presents anatomic lesions, defects and pathologies which might be erroneously interpreted as tumors of the urinary bladder, i.e. transitional cell carcinoma of the urinary bladder. The following lesions are discussed: 1) anatomic defects (including urachus remnants, ligaments that stabilize the bladder or cyst in the opening of the ureter into the bladder – ureterocele); 2) tumor-like lesions in the lumen of the urinary bladder (such as blood clots, fungus balls, stones or foreign bodies); 3) bladder wall pathologies (i.e. cystitis or endometriosis), focal decidual transformation of stromal cells or inflammatory pseudotumor; 4) lesions impressing on the bladder from the outside (the mesentery of the sigmoid colon, the bowel, pathological lesions in organs adjacent to the urinary bladder, inflammatory infiltration, vasogenic compression of the bladder, pelvic lipomatosis, pathological lesions of the pubic symphysis); 5) postoperative lesions. All these lesions may mimic carcinoma of the urinary bladder in sonography. Bearing this fact in mind is significant in establishing a diagnosis. Due to the malignant character of carcinoma of the urinary bladder and the need for aggressive surgical treatment, a correct diagnosis of this disease is essential for patients, particularly because the lack of adequate treatment and delayed treatment considerably affect prognosis.