The aim of the present study was to examine the impact of an antimicrobial prescribing protocol on clinical and economic outcome measures in hospitalized patients with community-acquired lower respiratory tract infection (LRTI). The study was performed as a prospective controlled clinical trial within the medical wards at Antrim Area Hospital, Northern Ireland. Data were collected on all hospitalized adult patients with a primary diagnosis of LRTI during the period December 1994 to February 1995 (normal hospital practice; control group; n = 112). After an LRTI management protocol (medical, microbiological and pharmacy staff) had been developed, all hospitalized adult patients with a primary diagnosis of LRTI over the period December 1995 to February 1996 formed the intervention group (treated according to the protocol; n = 115). The results showed a statistically significant impact of the protocol in terms of clinical and economic outcome measures. Patients treated using the algorithmic prescribing protocol had significant reductions in length of hospital stay (geometric mean 4.5 versus 9.2 days), iv drug administration (34.8% versus 61.6%), duration of iv therapy (geometric mean 2.1 versus 5.7 days) and treatment failures (7.8% versus 31.3%). Healthcare costs were also significantly reduced. The use of the protocol was a major factor in streamlining the prescribing of antimicrobial therapy for community-acquired LRTI and led to more cost-effective patient management.