Context: A postnasal drip (PND) or catarrh refers to the drainage of secretions from the paranasal sinuses or nose into the posterior nasal space and the oropharynx. A history of pharyngeal or postnasal mucus build-up may be at odds with the lack of other physical findings and the absence of systematic clinical data. The physiological basis and suitable treatments for PND have been insufficiently recorded in the medical literature. However, Iranian traditional medicine (ITM), which has a history of thousands of years, has discussed in detail the causes, origins, complications, and treatment of catarrh. Communication and cooperation between conventional and traditional medicine can lead to positive steps in solving the ambiguities related to catarrh. The present paper examines the origin of catarrh according to Avicenna and compares it with that described in conventional medicine. Evidence Acquisition: In this study, we examined a major resource of ITM, the Qanoon fi al-teb (The Canon of medicine), by Avicenna and the writings of prominent ancient scholars and physicians on the origins of catarrh. PubMed and Google scholar were also searched for information on PND and catarrh, and they were compared with the catarrh in ITM. Results: Physicians of ITM believe that the main substance in catarrh is discharged from the brain and that it is a connection between the brain and nasopharyngeal space. New scientific findings also confirm the relationship between cerebrospinal fluid (CSF) and catarrh, in common with that described by Avicenna thousands of years ago. Conclusions: Catarrh is a serious condition and requires more investigation. It is hoped that a joint study of conventional and traditional medicine can elucidate different aspects of catarrh.