Two patients had septic arthritis due to Pseudomonas aeruginosa. In one a heroin addict, the sternoclavicular joint was involved. The detection of precipitating antibodies to Pseudomonas led to the diagnosis of Pseudomonas arthritis. The infection was cured by surgical drainage and gentamicin sulfate therapy. The knee was involved in the second patient by spread into the joint from contiguous cellulitis or thrombophlebitis. The infection was not suppressed by parenteral administration of polymyxin B sulfate alone but was suppressed by parenterally and intraarticularly administered polymyxin and later by parenterally and intra-articularly administered gentamicin. Amputation was eventually required for cure of the infection. Pseudomonas is an unusual cause of septic arthritis, being found in only 2% of patients of a combined series. In treatment of Pseudomonas arthritis with either polymyxin or gentamicin, both parenterally and intra-articularly administered antibiotic should be used.