The classification of renal disease has been for years a thorny problem. The first classification, devised by Bright1, was based on the gross pathologic, the simple clinical findings and urinalysis available to him. Subsequently, Volhard and Fahr2, using the analytical chemical methods of their time and light microscopic examination of autopsy material, devised a classification that has been the benchmark in the diagnosis of renal disease for many years. However, this classification no longer suffices and the terms acute, subacute, and chronic glomerulonephritis are patently inadequate to describe the currently recognizable variation in the diseases affecting the kidneys. The difficulty with the more recent attempts at modifying the classification of renal disease derives from the fact that the authors have been held in the bonds of the traditional paradigm3,4. Our own dissatisfaction with the current assessment of renal pathologic material arose when two of us (G.E.S. and E.P.B.) were considering our series of biopsies (at that time 1200) collected over a period of years and described by different observers. Each observer, and at different times the same observer, emphasized or selected different features for comment, rendered a different "diagnosis" on the same case, and could not satisfactorily classify in any coherent scheme such things as homograft rejection reactions. As a consequence we began to search for a new and more effective method of obtaining and recording our observations with the aim of more useful analysis. Clearly, any method should provide, as nearly as possible, an objective, reproducible way of assessing abnormalities. The descriptive terminology must be as unambiguous as possible so that all workers in the field can clearly and accurately communicate their findings. Within this framework observations on the same or different patients at different times and at different places may be compared. The aims of this and its companion investigation were as follows: First, to provide a reasonable morphologic specification on renal biopsies of (a) the anatomic site involved and (b) the elementary or basic pathologic processes affecting each part of the kidney. We hoped to make this as free as possible of the many restraints of the traditional "diagnostic" categories. Second, we wished to introduce at least first order quantitative measures into the morphologic assay, so that some comparison could be made between morphologic and functional pathologic changes. It was our hope that in this way a new and more efficient scheme for establishing a diagnosis and a prognosis and developing a therapeutic regimen for renal disease might be initiated.