The optimal dosage of antibiotics for the treatment of patients with severe lower respiratory tract infections (LRTI) has not been determined but may be lower than is commonly administered at present. We have compared the efficacy of a low dosage of ceftazidime (1 g tds) with the more usual dosage (2 g tds) in a prospective, randomized study of the treatment of LRTI in seriously ill patients. Fifty patients on an Intensive Care Unit received one or the other regimen for 5 days; the demographic characteristics of the two groups were comparable. There was no significant difference in terms of clinical and microbiological response rates between the two regimens. Overall, clinical resolution was documented for 86% of patients, there was no change in 8% and 6% deteriorated. Microbiological eradication was achieved in 52% of patients from whom a pathogen was isolated (46% of all patients). We conclude that ceftazidime 1 g tds is effective treatment for severe LRTI in hospitalized patients.