Serial clinical and serological observations were made on a patient with necrotising polyarteritis associated with rheumatoid arthritis. Significant levels of circulating immune complexes, as determined by a C1q binding assay, were observed up to 2 years before the clinical manifestations of polyarteritis but rose abrumptly immediately before and concurrently with the onset of polyarteritis. Concomitant serial determinations of C3, latex fixation titres for anti-immunoglobulin, and patterns of fluorescence of antinuclear antibody afforded insight into the nature of these somplexes, as did clinical and serological response to glucocorticoid and cytotoxic therapy. Our data suggest that the antibody involved in the complex was of the IgG class and capable of complement fixation.