The heart is subject to involvement by primary (AL), senile (AS), and familial (AF) forms of amyloidosis, but the frequency, severity, and therapy of amyloid-related cardiac symptoms differ depending on the type of amyloidosis present. Endomycardial biopsy is a safe and reliable procedure for diagnosing cardiac amyloidosis, and immunohistochemical staining of routinely processed biopsy specimens can be performed to classify the type of amyloid present. However, whether or not the type can be determined from the histologic extent and pattern of amyloid deposition is unclear. Endomyocardial biopsy specimens from 100 patients with cardiac amyloidosis (74 AL, 22 AS, 4 AF) were examined, and the histologic extent and pattern of amyloid deposition were correlated with the amyloid immunophenotype. No difference in the extent of amyloid deposition was identified among the three types. Interstitial nodules of amyloid were more common in AS (82%) than in AL (50%, p = 0.0129), whereas vascular involvement was more frequently observed in AL (88%) than in AS (26%, p < 0.0001). Endocardial and interstitial pericellular deposition occurred with similar frequencies in both groups. Although statistically significant differences existed in the patterns of amyloid deposition, they did not allow reliable distinction between the different types in individual cases. Consequently, in older patients without serum or urinary light chains, immunohistochemical staining is recommended to distinguish AL from AS types of amyloid in cardiac biopsy tissues.