A study of 56 cases of non-infiltrating bladder tumour showed a striking difference in the distribution within the bladder of the primary and the recurrent tumours. More than three-quarters of the primary tumours were confined to a restricted area near a ureteric orifice whereas only one-fifth of recurrent tumours were found in this site. By far the commonest site for recurrent tumours was the postero-superior wall of the bladder. Forty-five recurrent tumours formed in the air bubble region, a site in which no primary tumour was found in the whole series. The postero-superior wall of the bladder is subject to mild trauma and abrasion by the tip of an endoscope and the air bubble region is the recipient of hot gas produced by the diathermy. These gas bubbles may also convey detached tumour cells to the recipient site. Local trauma may encourage tumour formation either by producing a raw surface which allows implanation of tumour cells, or possibly also by reducing local resistance to the primary carcinogenic factor.