In a clinical study the ultrasonic determination of the residual bladder volume is evaluated by applying several echographic methods described in the literature. 163 patients (age 3–89 years, 114 males and 49 females) and 13 students were examined. Only in the latter group the bladder volume was also determined by a planimetric method. The most practical method showed a good agreement (with a maximum deviation of 25%) between the ultrasonically measured volumes and the volumes measured by catheterization in 66% of the cases. This method makes use of the formula V_us = 0.5(V_t + V₁), where V_us = residual volume estimated by ultrasonography; V_t = volume measured by the computer using the largest transversal ultrasonic bladder section, and V₁ = volume measured for the largest longitudinal section). Factors that influence image quality and accuracy because of changes in the form of the bladder were analyzed. The magnitude of the residual volume determined the image quality and the accuracy above all: lower volumes give worse images and are less accurate. Other factors seem to be relatively unimportant. To avoid discomfort, caused by urine tract infection and urethral strictures, ultrasonic determination of the residual urine is recommended for clinical application.