T lymphocytes and monocyte/macrophages are prominent components of atherosclerotic lesions, and many of these cells are activated and secreting cytokines. To determine the role of these cells in the pathogenesis of atherosclerosis, we studied its development in T-cell-deficient mice fed a high fat atherogenic diet. Depleting euthymic mice of their CD4+ lymphocytes by 20 weekly injections of CD4 monoclonal antibodies reduced the mean area of their aortic lesions by approximately 70%. Similarly, the mean lesion area of T-cell-deficient nude (nu/nu) mice was 10% of the size of that of their heterozygote (nu/+) litter mates. Flow cytometric studies of splenic T cells and analyses of serum total and HDL cholesterol of these mice indicated that the differences in mean lesion areas among the experimental groups were most closely correlated with differences in splenic T cells content. These studies suggest that in these two models T lymphocytes contribute to the pathogenesis of early atherosclerotic lesions and that a further understanding of this phenomenon may provide future approaches toward the prevention and treatment of the disease.