The objective of this study was to investigate the spectrum of morphologic features in myocardial biopsy specimens from patients with cardiac immunocyte-derived (AL) amyloidosis. Cardiac involvement is the most important predictor of survival in AL amyloidosis. Myocardial biopsy remains the method of choice for diagnosing cardiac amyloidosis when noninvasive studies give equivocal results. Histologic, immunohistochemical, ultrastructural, and morphometric studies were made on myocardial biopsy specimens from 11 patients in whom the diagnosis of AL amyloidosis was based on the demonstration of a monoclonal immunoglobulinopathy and of amyloid deposits in tissues. Histopathologic study showed amyloid in 10 of the 11 biopsies. In one biopsy (Congo red negative), the diagnosis was made by ultrastructural identification of amyloid fibrils. In all patients, the deposits formed perimyocytic layers that measured up to 18 microns in thickness. These layers formed along the basement membranes, which were partially preserved in 5 patients and unrecognizable in 6. Interstitial nodular deposits were also present in 5 patients. Immunohistochemical studies for the characterization of the proteins in the amyloid deposits were diagnostic in 1 patient and confirmatory in 10. Nodular deposits, thick perimyocytic layers of amyloid and small myocyte diameters were associated with shorter survival of the patients. Small-vessel involvement and myofilament loss occurred in all patients. In conclusion, myocardial biopsy serves to (1) establish the diagnosis of cardiac amyloidosis; (2) characterize immunohistochemically the proteins in the amyloid fibrils and (3) assess the degree of myocyte damage and atrophy.