Thirty-one patients with rheumatoid arthritis were consecutively studied for evidence of muscle involvement, using muscle biopsy and electromyography. The patients were initially separated into 4 clinical categories: inactive peripheral joint disease (6 patients); active peripheral joint disease (10 patients); systemic disease and a disproportionately elevated sedimentation rate for the degree of mild synovitis (SERD) (11 patients); or elevated creatinine phosphokinase level (4 patients). In addition to routine histology, muscle tissue was examined for de novo synthesis of IgM and IgM rheumatoid factor, and by indirect immunofluorescence for the presence of immunoglobulin and complement deposits. Our results indicate that: muscle fiber necrosis occurs frequently in patients with rheumatoid arthritis; rheumatoid myositis, defined as muscle fiber necrosis and mononuclear cell infiltration, is a distinct entity and occurs particularly in patients with SERD or an elevated creatinine phosphokinase level; and only muscle from patients with rheumatoid myositis exhibited de novo synthesis of rheumatoid factor and significant quantities of IgM, indicating that local immune events may be important in the pathogenesis of this entity.