Thirteen cases (in twelve patients) of septic arthritis complicating rheumatoid arthritis are reported. One ankle, one metacarpopophalangeal joint, one shoulder, and ten knees were involved. Staphylococcus aureus was cultured from twelve joints and Escherichia coli, from one. Treatment consisted of repeated needle aspirations in two patients, arthrotomy with Penrose drainage in six, and arthrotomy with through-and-through irrigation in four. Needle aspiration was the least effective therapy. The authors recommend as the treatment of choice: systemic antibiotic therapy and immediate arthrotomy followed by through-and-through irrigation with fluid containing the appropriate antibiotics.