The aim of this study was to identify the spectrum of abnormalities revealed on high-resolution computerized tomography (HRCT) in patients with ankylosing spondylitis (AS), to compare findings with those of plain radiography and pulmonary function testing (PFT), and to look for correlations between lung involvement and AS severity. We prospectively studied 55 consecutive patients with a diagnosis of AS according to the modified New York criteria who attended our department over a period of 2 years. All patients had a detailed rheumatological examination and underwent plain chest radiography, chest HRCT and PFT. HRCT revealed abnormalities in 29 patients (52.7%), whereas plain chest radiography was abnormal in only 2. Abnormalities consisted of interstitial lung disease (ILD) ( n=4), apical fibrosis ( n=5), emphysema ( n=5), bronchiectasis ( n=4), ground glass attenuation ( n=2), and non-specific interstitial abnormalities ( n=26). Only apical fibrosis and bronchiectasis were statistically more frequent with increasing disease duration (significant trend chi(2)test, p=0.0029 and 0.028, respectively). PFT showed a restrictive process in 19 patients (34.5%). No correlation was noted between HRCT and PFT, nor with AS symptomatic and structural severity parameters. However, there was a statistically significant correlation between PFT and AS symptomatic and structural severity parameters. In conclusion,: this study confirms that the chest HRCT of patients with AS showed a great number of abnormalities undetectable by standard X-rays. The high incidence of lung abnormalities emphasizes the importance of excluding such a diagnosis in patients with AS even without respiratory symptoms.