Twelve patients with pneumococcal arthritis are described. Seven of the 12 patients had underlying diseases which predisposed them to pneumococcal infections; five were alcoholics and two had hypogammaglobulinemia. Five patients had pre-existing joint disease prior to the onset of septic arthritis. Seven patients had co-existent pneumococcal infection, including meningitis and/or endocarditis in five. The other five patients had pneumococcal arthritis without evidence of other foci of pneumococcal infection. With penicillin therapy and drainage of the purulent joint fluid (by needle aspiration in four and surgical drainage in seven), the function of the involved joint returned to normal or to the previous baseline level in all but one patient.