We report a case of composite pheochromocytoma/ganglioneuroma arising in a background of diffuse and nodular medullary hyperplasia in the adrenal gland of a 34-year-old man with multiple endocrine neoplasia 2a (MEN 2a). Cells were histologically classified as chromaffin or chromaffin-like (small typical-appearing pheochromocytoma cells), neuron-like (possessing ganglion cell morphology), and intermediate. We speculate that these cell types may represent a spectrum of differentiation of a neoplastic clone, with the intermediate cells representing a transitional stage between chromaffin cells and neurons. All three cell types in the composite tumor and all chromaffin cells in both nodular and nonnodular areas of the remaining medulla were strongly immunoreactive for tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis. In contrast, neuron-like cells (and to a variable extent intermediate cells) displayed selective loss of expression of phenylethanolamine-N-methyltransferase (PNMT), the enzyme that synthesizes epinephrine. Proliferative activity of the composite tumor and both the nodular and nonnodular medulla was studied by staining for the endogenous cell proliferation antigen Ki-67, using monoclonal antibody MIB-1. MIB-1 labeling was highest in Schwann cell areas of the composite tumor, followed by chromaffin-like cells in the composite tumor and in the separate nodules. Labeling was absent in neuron-like cells, consistent with the cells' postulated status as terminally differentiated derivatives of a chromaffin cell precursor, and was highly variable in nonnodular areas of the medulla. The latter observation suggests topographical variation in signals that drive chromaffin cell proliferation in MEN.