A patient is reported with a six-year history of seronegative inflammatory arthritis, lymphadenopathy, and a rash. Many investigations, including repeat jejunal and synovial needle biopsies, failed to establish the diagnosis. Eventually right hip destruction led to arthroplasty. Synovial membrane obtained at operation was examined by electron microscopy and organisms were found with the morphological characteristics of those in patients where the diagnosis was firmly established as Whipple's disease. We propose that Whipple's disease should be seriously considered as a rare but distinct and treatable possibility in the differential diagnosis of seronegative inflammatory arthritis.