The purpose of this paper is to emphasize the importance of information obtained by renal biopsy in the diagnosis, prognosis, and therapy of patients with renal disease. Because controversy persists regarding the value of renal biopsy as an aid in determining prognosis and in choosing appropriate therapy, there has been some reluctance to use it early after the onset of obvious signs, symptoms, and laboratory findings indicative of renal disease with or without involvement of other organs. Although all such patients may not benefit from the information provided by a proper biopsy, we will illustrate some of the characteristic histologic details found in specific circumstances in our experience where the biopsy has been particularly helpful in reaching a diagnosis, in assessing prognosis, and in choosing therapy.