A patient with Whipple's disease has been followed up for 4 years. Primary involvement was limited to the small intestines, and accumulation of periodic acid-Schiff-positive material, containing typical more or less intact bacillary bodies, was demonstrated within macrophages of affected tissue. After initial oxytetracycline treatment and clinical remission, the patient relapsed, with multiorgan affections. The antibiotic regimen was changed to chloramphenicol, followed by continuous trimethoprim-sulfamethoxazole. Flow cytometric studies showed persisting impairment of monocyte and macrophage intracellular degradation of bacteria during all the 4 years tested. After relapse, reduced activity of several brush border enzymes was demonstrated in distal duodenal biopsy specimens. After 17 months of continuous trimethoprim-sulfamethoxazole therapy complete clinical remission, regression of histopathologic abnormalities, and restoration of duodenal enzyme activities had occurred. The results demonstrate a persisting dysfunction of mononuclear phagocytes from a patient with Whipple's disease, suggesting a primary abnormality of cell-mediated immunity which may promote the susceptibility to the causative bacillus.