This report shows that the urethral lumen cross-sectional shape is complex but slit-like and it is suggested that this shape is related to function. A radiological method for determining urethral shape during micturition has been developed and compared with histological studies. The lumen is markedly non-circular even during micturition. Simple theoretical analysis of preliminary shapes shows that current concepts of urethral flow are incomplete and urethral pressure profile measurements require re-evaluation. The function of accepted sphincter mechanisms requires a slit-like lumen. Lumen shape is a fundamental property of the urethra, the clinical importance of which has yet to be explored.