Hypersensitivity pneumonitis is a complex interstitial lung disease caused by exposure to an inhaled antigen. Most recent guidelines proposed to categorize patients as having fibrotic or non-fibrotic subtype, mainly because the important implications for prognosis and treatment. Our focus is based on the relation between HP and patients? smoking habits. Since 70?s HP has been considered as a non-smoker lung disease, since it occurs more frequently in never smokers, although progression to fibrosis has been related to smoking exposure.

We here review this concept by a retrospective analysis involving 45 patients (24 females and 21 males) coming from Respiratory Unit of IRCCS Policlinico San Matteo in the last 10 years in acute and outpatients? settings.  The main age at diagnosis was 61.5 years. For most patients the antigen exposure was documented, in 6 cases was unknown.  In 19 out of the 45 cases, the diagnosis was confirmed histologically; in the remaining cases diagnosis was reached by matching imaging, biochemical and clinical data and after multidisciplinary discussion of each case. 29 out of the 45 cases, evolved in lung fibrotic disease.  Notably, although the significant percentage of smokers in the whole cohort analyzed, those patients harbouring fibrotic disease were never (15 cases) or past (11 cases) smokers, in 3 cases this data was unknown whereas only 1 patient currently smoked.

Overall our data suggest that HP could occur in smoker subjects, but that smoke seems not to play an active role in fibrotic evolution of the disease.  This unexpected finding deserves further analysis and validation for its relevant clinical, diagnostical and prognostic implications.