BACKGROUND Pulmonary Langerhans cell histiocytosis (PLCH) is an uncommon interstitial lung disease which can lead to serious respiratory failure. The correlation between high-resolution computed tomography (HRCT) findings and lung function have not been studied in depth. OBJECTIVE To assess the relationship between HRCT findings and lung function in PLCH. METHODS Since HRCT abnormalities in PLCH consist mainly of nodular opacities and cystic abnormalities, we determined semiquantitative scores of nodular profusion and cystic extent. We therefore assessed the relationship between HRCT abnormalities on one hand and lung function and gas exchange parameters on the other, in patients with PLCH. RESULTS In our series of 26 consecutive patients, we found no significant correlation between the score of nodular profusion and lung function or gas exchange parameters. The score of cystic extent showed a quite strong and significant correlation with FEV(1)/FVC (r = -0.62; p = 0.01), but also with PaO(2) (r = -0.69; p = 0.001) and carbon monoxide diffusing capacity (r = -0.60; p < 0.01). Furthermore, the patients with a predominant cystic pattern (n = 7) had the highest grade of dyspnea on exertion (p = 0.004), the lowest FEV(1)/FVC ratio (p = 0.02) and the lowest PaO(2) (p = 0.02) compared to patients with a predominant nodular (n = 12) or a mixed pattern (n = 7). CONCLUSIONS We conclude that in PLCH, the cystic extent on HRCT, but not the nodular profusion, correlates significantly with lung function abnormalities and impairment of gas exchange.