A case of Whipple's disease involving the small intestine, bone marrow and peripheral lymph nodes diagnosed one year after the initial arthritic symptoms by peroral jejunal biopsy and rapidly responding to treatment with sulphamethoxazole-trimethoprim, is described. The effectiveness of therapy was monitored by clinical evaluation of the patient's condition, repeated intestinal biopsies, determination of body weight, faecal lipid excretion and haematological values. Clear-cut clinical remission was induced promptly; and after nine months' treatment, the patient was in perfect health. The drug was well tolerated and folate deficiency did not occur. After four months of antibiotic therapy, bacterial capsular remnants could still be demonstrated in the bone marrow aspirate. The characteristic small bowel mucosal PAS-positive macrophages, visualized in the jejunal biopsy specimens, persisted in reduced number after nine months of treatment.