The results of treating chronic Pseudomonas aeruginosa osteomyelitis with cefsulodin at Rush-Presbyterian-St. Luke's Medical Center (RPSLMC) and eight other institutions are summarized. Eleven patients whose infections were proven by bone-biopsy culture were treated with cefsulodin at RPSLMC; one received two courses of treatment. Efficacy of therapy was evaluated for eight patients, all of whom had a polymicrobial infection. The average age of the patients was 52.3 years (range, 28-85). All had serious underlying illnesses or associated conditions. The mean inhibitory concentration of cefsulodin for the isolates of P. aeruginosa was 3.125 micrograms/ml (range, 0.78-6.25 micrograms/ml). Two patients received concomitant therapy with antibiotics not active against P. aeruginosa. Surgical debridement was performed in six of the eight patients. A favorable response was demonstrated in six of the eight patients. Follow-up for seven patients ranged from one week to 12 months, and in the eighth patient follow-up was 32 months. One patient relapsed twice. Seven possible complications of therapy were observed in five of the 11 patients who received cefsulodin; in three of these patients cefsulodin had to be discontinued. In studies of osteomyelitis conducted at other institutions, 10 of 14 patients for whom therapy could be evaluated had a favorable response to cefsulodin. Cefsulodin is a useful agent for the treatment of chronic osteomyelitis associated with P. aeruginosa.