Pulmonary complications pose a major clinical problem after bone marrow transplantation (BMT) or peripheral blood stem cell transplantation (PBSCT). The underlying pathophysiological mechanisms are under investigation. Twenty patients with infectious or non-infectious pulmonary complications after BMT or PBSCT underwent bronchoalveolar lavage (BAL) and tumor necrosis factor-alpha (TNFalpha), interleukin-10 (IL-10) and interleukin-18 (IL-18) mRNA expression was determined in BAL cells in comparison to 11 healthy volunteers. Patients were divided into two groups (infectious pneumonia, n = 14 or idiopathic pneumonia syndrome (IPS)/bronchiolitis obliterans (BO), n = 6). TNFalpha expression was significantly increased in both patient groups compared to the control (0.40 +/- 0.07 and 0.39 +/- 0.09 vs 0.15 +/- 0.05; P < 0.05; semiquantitative PCR analysis; mean +/- s.e.m.). IL-10 expression was significantly elevated the group of patients with infectious pneumonia in comparison to normal controls (0.15 +/- 0.06 vs 0.01 +/- 0.01; P < 0.05) but not in patients with IPS/BO (0.05 +/- 0.04; P > 0.05). However, IL-10 was not expressed in BAL cells of all patients and control individuals. IL-18 expression was significantly decreased in the both patient groups (1.47 +/- 0.24 and 1.79 +/- 0.63) in comparison to the control group (4.29 +/- 0.77; P < 0.05). Survival analysis showed a significant association between elevation of TNFalpha and poor prognosis (P < 0.05). These results highlight the immunoregulatory disturbances in the lungs after BMT/PBSCT.