The electron-microscopic and light-microscopic findings on jejunal biopsies from a case of Whipple's disease are described. Seven biopsies were obtained over a 3-year period. Rapid clinical recovery was accompanied by the disappearance of interstitial and intracellular bacteria from the lamina propria. The pathognomonic PAS-positive material on light microscopy corresponding to the phagocytic vacuoles in the macrophages of the lamina propria decreased markedly in amount but persisted throughout the 3-year-period. The sequential changes in the jejunal mucosa and in particular those occurring in macrophages are described and their relation to pathogenesis is discussed.