Histiocytosis is a rare cause of diffuse interstitial pneumonia. Its aetiology is not known and the outcome is often unsatisfactory. In 10 subjects in whom the diagnosis of histiocytosis X was established at CHU in Grenoble, 7 could be followed up. We report the clinical, functional, radiological and high resolution tomographic data on these 7 subjects suffering from histiocytosis X and followed up for a period between 3 and 13 years (8 +/- 3 years). All the subjects were symptomatic at the time of diagnosis. The clinical outcome amongst the 10 subjects included 2 patients who died, 2 who stabilised, 1 worsened and there were 4 clinical cures, 1 patient was lost to follow up. The respiratory function tests of the 7 subjects who were followed up was characterised by the appearance of airflow obstruction and a significant fall in the DLCO/VA (KCO) of 68 +/- 17% of the predicted value to 43 +/- 13%. An analysis comparing the initial pulmonary radiography to the current films showed that the reticular nodular lesions tended to progress to a diminution in size, while there was an increase in the reticulation. The high resolution computed tomography was confirmed as better technique than pulmonary radiography in detailing the cystic and nodular lesions. This was the only examination which correlated with DLCO/VA (R: 0.8; p: 0.018). A progressive model for computed tomographic lesions is proposed. Computed tomography appears to be the key examination, orientating the diagnosis based on the association of the nodules and cysts and enabling a better appreciation of the severity of the pulmonary disease taking account of its excellent correlation with diffusion.