Forty-three patients with classical multiple myeloma were studied to assess the prevalence and characteristics of amyloid arthropathy using clinical, radiological, and histological methods. Two patients were found to have amyloid arthropathy, and a third case is described in detail. Complement and cryoprecipitate analysis of the synovial fluid, and electronmicroscopy of the synovium, synovial debris and cartilage were undertaken in an attempt to shed further light onto the pathogenesis of what has hitherto been regarded as a rare complication of multiple myeloma. Complement components were not depressed and no evidence of specific light chain containing immune complexes was found in synovial fluid cryoprecipitates. Histochemical and electron microscopic localization of amyloid in perichondrocytic lacunae as well as in synovial fluid debris, synovium and the articular cartilage surface suggest the possibility that chondrocytes and synovial macrophages may share a role in processing immunoglobulin components as a prelude to the formation of amyloid fibrils. Amyloid arthropathy occurs in about 5 per cent of patients with multiple myeloma. The clinical picture can resemble rheumatoid arthritis with median nerve compression in the carpal tunnel and symmetrical arthritis of the wrists and small joints of the hands. Diagnosis can be established by examination of Congo red stained synovial fluid sediments under polarized light even in the absence of other clinical features of amyloidosis and when rectal biopsy is negative.