The renal response to deposition of monoclonal light chains represents a spectrum of pathologic changes that can be divided into glomerular or tubulo-interstitial processes. Involvement of the tubulo-interstitium can include activation of the proximal tubule, proximal tubule injury/cell death, and cast nephropathy. In these diseases, the culprit is not the intact immunoglobulin protein but instead the immunoglobulin light chain. Recent non-invasive tests, including immunofixation electrophoresis or quantification of serum free light chains, have increased the sensitivity for detection of an abnormality in circulating free light chains and are invaluable ancillary tools, but short of renal biopsy, the diagnosis of these diseases can prove challenging. A description of the pathobiology and overview of the approach to management of these light chain-mediated renal lesions is provided. Copyright © 2007 S. Karger AG.