The thymic epithelium was compared in weanling male and female CBA/J mice when fed ad libitum and when subjected to severe food intake restriction for 14 days. The restriction protocol elicited predominantly a metabolic response to caloric deficit rather than to protein deficiency. Electron microscopy revealed intracytoplasmic accumulations of large, circular, homogeneously electron-dense profiles (with no limiting membrane) in a high proportion of cortical and medullary epithelial cells of thymuses from restricted mice, but not from controls. The electron-dense material was not preserved in the absence of osmium. Thin-layer chromatography (TLC) indicated elevated levels of free and esterified cholesterol, particularly the latter, in whole thymus extracts of restricted mice. Measurements of total cholesterol levels in the thymic extracts were consistent with the results obtained by TLC. In addition, cryostat sections of thymuses from restricted mice, but not from controls, exhibited numerous stained foci throughout the cortex and medulla when treated with oil red O (a general neutral lipid stain) or by the Schultz procedure which is specific for cholesterol. Collectively the results suggest accumulations of cholesteryl esters, together with some free cholesterol, as non-membrane-bound droplets in the cytoplasm of thymic epithelial cells of undernourished mice. It is also of interest that the lipid-laden epithelial cells exhibited none of the cytoplasmic vacuoles observed in controls and believed to be important in thymic hormone secretion. This work provides the first direct evidence of thymus epithelial abnormalities in severe protein-energy malnutrition.