The thymus is the major site of T cell maturation; extensive proliferation, differentiation, and apoptosis occur in this organ. During mammary tumorigenesis, there is a profound involution of the thymus associated with a severe depletion of the most abundant subset of thymocytes, CD4+8+ immature cells. Experiments to investigate the mechanism of loss of the CD4+8+ population indicated that there was no increase in the systemic levels of glucocorticoids, no loss of bone marrow precursors, and no decrease in precursor seeding of the thymus. Likewise, no enhanced emigration of thymocytes from the thymus to the periphery was observed in tumor-bearing mice. A slight increase in apoptosis was found in tumor bearers' thymi, but there was no apparent decrease in the proliferation of early thymic precursors CD4-8- cells. Importantly, severely altered levels of subpopulations of the CD4-8- precursors, consistent with an arrest in differentiation at an early stage of development, were detected. Moreover, thymic stromal cell function appeared to become impaired during tumorigenesis, possibly due to the action of tumor-derived factors. Thus, downregulation of cell-mediated immune functions occurring at late stages of the disease may be causally related to the thymic involution occurring during mammary tumorigenesis.