The proposition that post-nasal drip (PND) is a common cause of cough in childhood is controversial. The concept of PND as a common cause of cough is derived primarily from the adult literature. The definition of what constitutes PND is variable and it is unclear whether it is a symptom, a sign or both. Examination of the specificity of symptoms and signs for the diagnosis of PND syndrome further confuses the issue. A definitive diagnosis of cough induced by PND cannot be made from history and physical examination alone. The concept is inconsistent both with the meaning of the word 'drip' and the science of rheology. The most plausible explanation for the occurrence of cough in children identified with increased post-nasal secretions is that both reflect co-existent airways pathologies. In considering causes of chronic cough in childhood, it is now time to abandon the concept of PND and the associated PND syndrome.