In this study the efficacy and cost-effectiveness of i.v. ceftriaxone 1 g once daily (CTX) was compared with standard i.v. antibiotic treatment (STD) for lower respiratory tract infections (LRTI). STD was given according to the guidelines of the American Thoracic Society and consisted of either cefuroxime 1500 mg three times daily (q8h), amoxicillin/clavulanic acid 1200 mg q8h or ceftriaxone 2 g once daily; each with or without a macrolide. After a minimum of 5 days i.v. therapy, patients could be switched to oral therapy. One hundred patients were enrolled in the study; 52 patients received CTX and 48 STD. Groups were comparable with respect to demographic and baseline characteristics. Seventy patients had a confirmed diagnosis of pneumonia. Twenty-nine patients had a severe type I exacerbation of chronic bronchitis. In one patient the diagnosis of LRTI could not be confirmed. In approximately 50% of the patients a microbiological diagnosis could be made. The most important isolated pathogens from sputum and blood were (positive blood cultures in brackets): Streptococcus pneumoniae 14 (9) and Haemophilus influenzae 16. Mean duration of i.v. therapy was 7.4 days in both groups. Average duration of hospitalisation was 15.0 days for CTX patients and 15.9 days for STD patients. Overall cure and improvement rate at the end of treatment was 47 (90%) for patients receiving ceftriaxone 1 g compared to 37 (77%) for patients receiving standard therapy. Pathogens were eradicated or presumed to be eradicated in 84% of the CTX patients and in 76% of the STD patients. Mean total costs per treatment were lower for CTX than for STD treatment: NLG 169 versus 458. These results show, that i.v. ceftriaxone 1 g once daily is as effective as standard therapy in the treatment of LRTI and that its use reduces treatment costs, in view of the multiple daily dosing regimens of most standard therapies.