Commercial techniques are available to calculate effective renal plasma flow (ERPF) or glomerular filtration rate (GFR) based on the percent injected dose in the kidney 1-2 or 2-3 min post-injection; renal depth is estimated by the Tonnesen equations. Since the Tonnesen equations were derived from ultrasound measurements obtained at an oblique angle in sitting patients, we compared the renal depths obtained from the Tonnesen equations with the renal depth measured by computed tomography in supine patients, the most common position for radionuclide renography. The renal depth, height, weight, age and sex were determined for 126 patients undergoing CT scanning. Patients with obvious renal or abdominal pathology were excluded. The Tonnesen equations significantly underestimated renal depth. Using stepwise linear regression analysis, we derived a set of equations based on age, height and weight and applied these prospectively to a new set of 75 patients. In addition, a second set of equations were derived for the new data. There was no difference in the results for the two equations. We then pooled both studies and derived a combined set of equations: right renal depth (mm) = 153.1 weight/height + 0.22 age + 0.77 and left renal depth (mm) = 161.7 weight/height + 0.27 age - 9.4, where weight is in kilograms and height is in centimeters. The correlation coefficients were 0.81 and 0.83 for the right and left kidneys respectively with standard errors of the estimate of 10.2 and 10.1 mm. These equations provide a much better estimate of renal depth in the supine patient than the Tonnesen equations.