Patients with hospital-acquired lower respiratory infections pose both diagnostic and therapeutic challenges. Such infections are commonly seen in critically ill patients. When nosocomial pneumonia is suspected, treatment is generally initiated with broad-spectrum antibiotics before culture results become available. The usual therapeutic regimen includes an aminoglycoside with or without a beta-lactam agent. In a clinical efficacy study of a single agent, ceftazidime, in the treatment of 20 adults with hospital-acquired lower respiratory infection, 18 patients showed clinical improvement with ceftazidime therapy and pathogens were eradicated in 11. Therapeutic failures occurred in two patients who received empiric therapy prior to the isolation of pathogens resistant to ceftazidime. The median minimal inhibitory concentration of ceftazidime for the isolated pathogens was 0.78 micrograms/ml. Of the 15 patients infected with Pseudomonas aeruginosa, 14 showed a favorable clinical response. Therapy-limiting side effects occurred in two patients and bacillary resistance developed in one patient. The efficacy and safety of ceftazidime in the treatment of hospital-acquired pneumonias were comparable to results previously demonstrated for amikacin, cefotaxime, and imipenem in studies conducted at our institution. In studies reported in the literature, 44 of 51 patients (86 percent) with nosocomial pneumonia who were treated with ceftazidime had a favorable clinical response to therapy. The patients included in these studies were neither neutropenic nor commonly bacteremic, and none had cystic fibrosis. Ceftazidime appears to be a useful agent in the treatment of selected patients with nosocomial pneumonias, including those due to P. aeruginosa.