To detect the presence of amyloid, we studied synovial fluid samples from 7 patients who had biopsy-proven amyloidosis and clinical arthropathy and from 98 patients who had other forms of arthritis. Centrifuged sediments from synovial fluids of all 7 patients with amyloid and clinical arthropathy were positive on staining with Congo red, whereas the 98 others were negative. Under polarized microscopy, the amyloid appeared as an amorphous material with strong Congo red uptake and typical apple-green birefringence. These findings indicate that analysis of synovial fluid sediments with Congo red may represent a simple, yet sensitive, test for the diagnosis of amyloid arthropathy.