Various immunological processes are suspected of having some relationship with the onset of diffuse panbronchiolitis (DPB). Therefore, the cellular and humoral immunity of patients with DPB were studied. Sera of patients with DPB showed a high frequency of positive RA factor and high titer of cold agglutinin. Delayed skin tests with phytohemagglutinin (PHA) and purified protein derivative (PPD) in patients with DPB revealed normal responses to PHA as nonspecific mitogen and suppressed responses to PPD as specific antigen, although lymphocyte blastogenesis in vitro showed suppressed responses to PHA. The proportion of lymphocytes in bronchoalveolar lavage (BAL) fluid of DPB patients equaled that of normal controls. Furthermore, the CD4/CD8 ratio of T-cell subsets in BAL fluid of DPB patients were significantly lower than that of normal controls, although the CD4/CD8 ratio in peripheral blood of DPB were significantly higher than that in normal controls. The percentage of B-cells in the peripheral blood of DPB patients was hihger than that in normal controls. These data suggest that the activation of cellular and humoral immune mechanisms could play an important role in the pathogenesis of DPB.