Synovial fluid and membranes were studied in two patients with Whipple's disease. In the first, arthrocentesis revealed 28,350 white blood cells/mm3, 96% of which were polymorphonuclear leukocytes. A dense neutrophilic infiltrate, periodic acid-Schiff, positive macrophages and bacilliform structures were present in the synovial membrane. The synovial fluid and membrane findings in the second case, and during intercritical periods in the first case, showed mild to moderate nonspecific inflammation. Since polyarthritis and other systemic manifestations often precede the gastrointestinal features of Whipple's disease by many years, a closed synovial membrane biopsy should be obtained in the appropriate clinical setting.