BACKGROUND: Chronic cough is a common symptom that requires the systematic diagnostic approach for proper evaluation. Postnasal drip syndrome(PNDS), bronchial asthma, gastroesophageal reflux disease(GERD), and chronic bronchitis are among the common causes. This study was conducted to evaluate the spectrum and the frequency of the causes of chronic cough. METHODS: We prospectively evaluated 93 patients who had chronic cough despite normal chest radiographic finding. History and physical examination were done along with paranasal sinus radiograph, spirometry, bronchoprovocation test and 24-hours' ambulatory aesophageal pH monitoring as necessary. RESULTS: Forty-nine(52%) of the 93 patients had PNDS, 15 patients(16%) bronchitis, 10 patients(11%) asthma, 4 patients(4%) GERD, 7 patients(8%) both PNDS and asthma, 4 patients(4%) undiagnosed condition and 4 patients(4%) were taking ACE inhibitor. Sixty-nine percent of the patients with PNDS improved after follow up, 73% patients with bronchitis, 80% patients with asthma, 50% patients with GERD, 100% patients with both PNDS and asthma, and 100% patients with ACE inhibitor. CONCLUSION: PNDS was the most common causes of chronic cough. Bronchitis was the second and asthma the third in frequency. The etiology of chronic cough can be determined easily by history and physical examination, successful therapy initiated in most patients. The response to specific therapy also was important in evaluation of chronic cough.