A prospective study of the post-micturition radiograph in 539 consecutive intravenous urograms was performed. In only 1% of patients was a post-micturition residue demonstrated only on this radiograph and not on the control radiograph. The post-micturition radiograph helped in making a firm diagnosis in two out of six patients with vesico-ureteric junction calculi, which were suspected but could not be confidently diagnosed without it. It was also helpful in two out of 13 bladder patients with carcinoma, enabling a confident diagnosis to be established where the lesion had only been suspected. It would seem pertinent to restrict the post-micturition radiograph to cases of haematuria and possible vesico-ureteric calculi.