All published evidence on procalcitonin (PCT)-guided antibiotic therapy was obtained in trials where physicians knew that they were being monitored, possibly resulting in higher adherence to the PCT algorithm. This study investigates the effectiveness of PCT guidance in an observational quality control survey. We monitored antibiotic therapy and algorithm adherence in consecutive patients with respiratory tract infections admitted to the Kantonsspital Aarau, Switzerland, between May 2008 and February 2009. The results were compared to the site-specific results of the former ProHOSP study. Overall and more pronounced for patients with community-acquired pneumonia, the median duration of antibiotic treatment in this survey was shorter than the ProHOSP control patients (6 vs. 7 days, P = 0.048 and 7 vs. 9 days, P < 0.001). In 72.5% of patients, antibiotics were administered according to the prespecified PCT algorithm. No significant differences concerning adverse medical outcome could be detected. This study mirrors the use of PCT-guided antibiotic therapy in clinical practice, outside of trial conditions. If algorithm adherence is reinforced, antibiotic exposure can be markedly reduced with subsequent reduction of antibiotic-associated side effects and antibiotic resistance. The integration of the PCT algorithm into daily practice requires ongoing reinforcement and involves a learning process of the prescribing physicians.