In this prospective, randomized, placebo-controlled clinical study we aimed to investigate the clinical effect of a Cistus extract (CYSTUS052) in 160 patients with infections of the upper respiratory tract. The extract contains a high percentage of highly polymeric polyphenols. In cell culture and in a mouse model it exerts antiviral and antimicrobial activities. Principal active constituents of the genus Cistus are polyphenolic compounds. Plant-derived polyphenols have been shown to be strong antioxidants with potential health benefits. Various reports have appeared on the antiviral and antibacterial potential, including several reports describing the antiviral activity of polyphenols against influenza virus. Clinical studies on the effectiveness of Cistus incanus are scarce. Only one controlled application observation study demonstrated the effectiveness of a Cistus extract. The present randomised, placebo-controlled clinical study was designed to compare the symptom scores in patients with common cold treated either with CYSTUS052 or with placebo. A score of subjective symptoms decreased significantly over the course of treatment with Cistus, whereas treatment with placebo resulted in a less distinct decrease of symptoms. Among the inflammatory markers investigated, the C-reactive protein was mostly affected by Cistus and decreased significantly in the treatment group.