Recent studies have suggested that procalcitonin (PCT) is a safe marker for the discrimination between bacterial and viral infection, and that PCT-guided treatment may lead to substantial reductions in antibiotic use. The present objective was to evaluate the effect of a single PCT measurement on antibiotic use in suspected lower respiratory tract infections (LRTIs) in a Danish hospital setting. In a randomized, controlled intervention study, 223 adult patients admitted to the hospital because of suspicion of LRTI were included with 210 patients available for analysis. Patients were randomized to either PCT-guided treatment or standard treatment. Antibiotic treatment duration in the PCT group was based on the serum PCT value at admission. The cut-off point for recommending antibiotic treatment was PCT > or =0.25 microg/L. Physicians could overrule treatment guidelines. The mean duration of hospital stay was 5.9 days in the PCT group vs. 6.7 days in the control group (p 0.22). The mean duration of antibiotic treatment during hospitalization in the PCT group was 5.1 days on average, as compared to 6.8 days in the control group (p 0.007). In a subgroup analysis of chronic obstructive pulmonary disease patients, the mean length of stay was reduced from 7.1 days in the control group to 4.8 days in the PCT group (p 0.009). It was concluded that the determination of a single PCT value at admission in patients with suspected LRTIs can lead to a reduction in the duration of antibiotic treatment by 25% without compromising outcome. No effect on the length of hospital stay was found.