Cell proliferation, an important mechanism of atherosclerotic plaque growth, occurs among smooth muscle, inflammatory cell, and other cell types. We have identified different topographical patterns of cell proliferation in human carotid plaques, based on cell type. Cell proliferation was determined with an antibody to the proliferating cell nuclear antigen (PCNA), combined with cell type-specific antibodies. Despite low levels of overall proliferative activity, the intima displayed more proliferative activity than the underlying media (1.61 +/- 0.35% in intima versus 0.05 +/- 0.03% in media; P < 0.01). The preponderant proliferative cell type in the intima was the monocyte/macrophage (46.0% of PCNA-positive cells), with a minority being smooth muscle alpha-actin-positive (9.7%), microvascular endothelial (14.3%), and T cells (13.1%). Smooth muscle cells were the dominant proliferating cell type in the media (44.4% of PCNA-positive cells versus 20% endothelial cells, 13.0% monocyte/macrophages, and 14.3% T cells). Within the plaque, foam-cell-rich regions mostly displayed proliferation among macrophages (66.5%), whereas in vascularized fields PCNA positivity was almost equally shared by endothelial cells (23.8%), monocyte/macrophages (26.3%), smooth muscle alpha-actin-positive cells (14.0%), and to a lesser extent, T cells (8.2%). Logistic and linear regression analyses also demonstrated that location in foam-cell-rich regions was a significant predictor of proliferation only among monocyte/macrophages, whereas location in vascularized regions was a good predictor of PCNA positivity among both inflammatory and noninflammatory cells. These different patterns of cell type proliferation suggest possibly different distributions of putative responsible growth regulatory factors in human atherosclerosis.