BACKGROUND Sputum induction is a non-invasive procedure for measuring inflammatory processes of the lower respiratory tract. The aim of this study was to establish sputum cell counts and differentials in patients after lung transplantation (LTx), with or without chronic transplant rejection. METHODS Sputum induction was performed in 41 LTx patients (25 single LTx (sLTx), 16 double LTx (dLTx) and 15 healthy non-smoking volunteers. Sputum was processed according to standard protocols. Total cell count was calculated as mean (SE) cells x 10(6)/ml sputum and cell differential (%) was evaluated after staining. Cellular profiles were correlated with lung function. RESULTS Total sputum cell counts were increased in sLTx (9 (1.9) cells x 10(6)/ml, p=0.01) and dLTx patients (7.2 (1.5) x 10(6)/ml, p=0.01) compared with healthy controls (2.6 (0.6) x 10(6)/ml). There was also a marked sputum neutrophilia in both patient groups (59 (6)% and 62 (6)%, respectively, p<0.001 v controls). Moreover, in both sLTx and dLTx patients with chronic transplant rejection there was an increased number of sputum neutrophils compared with patients with normal graft function (p<0.05 both comparisons), and neutrophils were inversely correlated with lung function (forced expiratory volume in one second (FEV(1)) % predicted): sLTx, r=-0.61, p=0.001; dLTx, r=-0.75, p=0.001, respectively). Sputum lymphocytes and eosinophils were similar in both groups. No relevant side effects occurred during sputum induction. CONCLUSIONS Sputum induction is a safe and non-invasive tool for monitoring lower respiratory tract inflammation in LTx patients. Both sLTx and dLTx patients with chronic rejection had increased sputum neutrophils compared with patients with normal transplant function. These data support findings of other authors highlighting a possible role for neutrophils in the pathogenesis of chronic transplant rejection.