This is a quantitative study of the effect of a full and empty bladder on the position of the uterus, ovaries and bladder relative to the lumbar spine. Data are used to estimate the difference in radiation dose to these organs from performing a lumbar spine CT investigation or a lateral lumbar spine radiograph with a full bladder compared with an empty bladder. 12 women of child-bearing age underwent pelvic magnetic resonance scans with full and empty bladders. The positions of the uterus, ovaries and bladder were matched with the radiation dose distribution that would have occurred either side of the inferior boundary of the CT scan volume and the lateral lumbar spine radiograph. These radiation dose profiles were measured on phantoms using a combination of ionization chambers and thermoluminescent dosemeters. When the bladder was emptied, the mean position of the endometrial cavity fundal tip moved from 4.1 cm to 6.1 cm inferior to the centre of the L5/S1 disc space, and from 0.87 cm to 1.12 cm anterior to the centre of the L5/S1 disc space. This movement on micturation would have reduced the mean dose to the uterine internal fundal tip during a pelvic CT scan from 6.8 mGy to 3.9 mGy, which represents a mean reduction of 43% (range 12-67%). The mean dose from a lateral lumbar spine examination would have been reduced from 197 microGy to 126 microGy. The change in ovary position results in the mean ovary dose being reduced by 48% for the lumbar spine CT scan and by 43%) for a lateral lumbar spine radiograph. When the bladder was emptied, the average position of the bladder wall moved from 7.2 cm to 10.3 cm inferior to the L5/S1 disc space. This change in bladder position reduces the mean dose to the wall of a full bladder from 5.7 mGy for a CT scan and 114 microGy for a lumbar spine radiograph to 2.2 mGy and 42 microGy, respectively, for an empty bladder.