The aim of this study was to determine the clinical and bacteriological efficacy and safety of piperacillin-tazobactam (PT) (4g/500 mg IV tid) in the treatment of 107 adult patients with lower respiratory tract infections (LRTI) requiring hospitalization. Patients included were 66 men and 41 women with a mean age of 55.2 years (range 18-89), enrolled from Mexican (6) and Argentinean (5) hospitals. Ninety-nine clinically evaluable patients (92.5%), 87 with pneumonia and 12 with bronchitis, were treated for a mean period of 9.3 and 7.3 days, respectively. Response to treatment was favorable in 94.3% cases with pneumonia and 100% of cases with bronchitis; 86 cases (80.3%) were bacteriologically evaluable, 77 with pneumonia (eradication 74, persistence 1, superinfection 2), and 9 with bronchitis (eradication in all). Streptococcus pneumoniae was recovered in 24, Klebsiella pneumoniae in 21, Staphylococcus aureus in 8, Haemophilus influenzae in 7, Pseudomonas aeruginosa in 5, Enterobacter spp. in 6, Escherichia coli in 6 and other organisms in 12. Toxicity or intolerance were not observed. Our data suggest that PT is a reliable therapy for severe LRTI.