Four patients whose rheumatoid arthritis (RA) was complicated by staphylococcal arthritis were identified. All patients had active, long-standing disease with destructive changes. Affected joints included hip (two patients), knee (one patient), and shoulder (one patient). Pain and loss of motion in the affected joint were prominent, but toxic features of pyogenic infections--hectic fever, chills, sweats, local warmth, or erythema--were conspicuously absent. Two patients had moderate fever and three patients had mild leukocytosis. No patient was leukopenic. When present, fever was attributed to infected decubiti or urinary tract infection and treated with antibiotics. Therapy with corticosteroids and nonsteroidal antiinflammatory drugs (NSAIDs) probably masked symptoms and delayed the correct diagnosis. Purulent synovial effusions were discovered serendipitously--during arthrography (knee), attempted Girdlestone procedure (hip), and aspiration prior to steroid injection (shoulder). Sepsis was included in the preoperative diagnoses only once (hip). Prior instrumentation (aspiration or injection) of the affected joint was not a feature in any patients, although one patient had undergone insertion of a knee prosthesis one year prior to sepsis. Infectious organisms were Staphylococcus aureus in three patients and Staphylococcus epidermidis in one. Severe sequelae ensued in three of four patients: death from recurrent sepsis (one patient), loss of prosthesis leading to knee arthrodesis (one patient), and protracted sepsis with additional pyarthrosis (one patient). The only patient to regain preseptic joint function (shoulder) had not been on long-standing corticosteroids. Pyarthrosis must be considered in RA patients with unusually painful or stiff joints even in the absence of toxic symptoms.