The Trigonal canal is a new roentgen anatomical structure which develops and exists only during the act of voiding. It was discovered incidentally during a roentgen study of the lower urinary tract undertaken to evaluate bladder neck and urethral pathology. High quality, sequential films of the bladder base during voiding are required to demonstrate it. The structure has escaped recognition in the past primarily because employed methods of cystourethrography have consisted of either a single voiding film or sequential filming which, if performed, usually did not include late voiding. Kjellberg et al. (2) include several examples of the trigonal canal in their excellent treatise on the lower urinary tract, but ignore it as an anatomical entity. The purpose of this paper is to describe the roentgen anatomy and physiology of the bladder base as they pertain to the formation of the trigonal canal and to correlate roentgen and gross anatomy where possible. Material and Methods Sequential cystourethrography, wi...