Herbal preparations based on Echinacea species are well known for their stimulatory and modulatory effects on the cellular and humoral immune system. The current investigation was designed to obtain scientific data on the clinical efficacy of Echinacea supplement in companion animals. This randomised, double-blind placebo controlled multi-center trial in the veterinary practice was performed using a standardised and stabilised root powder of Echinacea purpurea and a placebo powder in 63 (30 and 33 per group) dogs presenting with kennel cough complex. Leading clinical symptoms to be investigated were ocular secretion, nasal secretion, reddening of the pharyngeal mucosa, enlargement of lymph nodes, difficulty in swallowing/sore throat, retching, vomiting, panting, tracheal respiratory sounds, bronchial respiratory sounds, dyspnoea, sneezing and dry cough. Following 14 days of dietary supplementation, "severity of disease" was reduced faster in the Echinacea group. At the trial's end 56% of animals in the Echinacea group and only 26% in the placebo group were without any clinical signs (p=0.02). Efficacy of Echinacea was accordingly assessed by the investigators as "very good" in the majority of dogs in that group. During the study there was a significant improvement (p<0.05) of typical canine symptoms and complaints, e.g. for nasal and ocular secretions, tracheal respiratory sounds and enlarged lymph nodes in the Echinacea group, which were not evident in the placebo group. There were no relevant adverse effects suspected to be related to either of the study preparations. The stabilised powdered root of Echinacea purpurea was shown to be an effective and safe immunomodulatory dietary support in treatment of acute and chronic conditions of kennel cough complex in dogs.