In the past researchers have used an estimate of one million as the number of glomeruli in each human kidney. However, recent work on excised kidneys has demonstrated a large variation in glomerular number from one person to another (330,000 to 1,400,000) per kidney. Theoretically an in situ estimate of glomerular number could be obtained if renal cortical volume, volume density of glomeruli per cortex [Vv(glom/cortex)] and mean glomerular volume are known. We used a dog model to demonstrate that an accurate estimate of cortical volume could be obtained in situ using magnetic resonance imaging (MRI). Vv(glom/cortex) and mean glomerular volume were obtained from needle biopsies. An independent and more direct method (the fractionator) was used to validate the estimate of glomerular number obtained using MRI and renal biopsy. On average there was very good agreement between the fractionator method (379,000 +/- 40,000) and the MRI/renal biopsy method (376,000 +/- 108,000) for the 10 dog kidneys measured; however we found up to a 36% difference between the two methods in an individual kidney. Nonetheless, the estimate from the MRI/renal biopsy method has more precision than the assumption that there are one million glomeruli per human kidney.