Primary or AL amyloidosis occurs in patients with monoclonal plasma cell-related disorders and is typically associated with the systemic deposition as amyloid fibrils of the light-chain portion of the immunoglobulin molecule. Recently, the discovery that heavy chains could be involved in amyloid formation led to the designation of this type of disease process as AH amyloidosis. We have now identified a second example of heavy chain-associated amyloidosis in a patient (MAD) who had a serum IgG monoclonal gammopathy and Bence Jones proteinuria. In this case, the renal and splenic amyloid deposits consisted solely of the VH-D-encoded portion of the heavy polypeptide chain, in contrast to the first case, where the amyloid contained an immunoglobulin component composed of the entire heavy-chain variable and third constant domains. In this respect, the chemical composition of the amyloid protein MAD differed not only from that of the first reported case of AH amyloidosis but from all other structurally abnormal components found in patients with heavy chain-associated disease. The discovery that certain forms of heavy chains, as well as light chains, can form amyloid provides further information on the chemical basis of amyloidogenicity and the diverse nature of this disease.