BACKGROUND: Eosinophilic lung diseases comprise a group of heterogeneous pulmonary disorders linked by increased eosinophils in bronchoalveolar lavage fluid (BALF). There is supporting evidence that natural killer (NK) cells participate in the regulation of eosinophilic inflammation.OBJECTIVE: Our aim was to investigate the relationship between eosinophils and NK cells in BALF in patients with different interstitial lung diseases (ILDs) focusing on eosinophilic pneumonias.METHODS: Of 114 patients who presented with increased BALF eosinophils (>5%), 74 patients were classified into the following groups: 27 had eosinophilic pneumonia (EP), 17 had idiopathic pulmonary fibrosis (IPF), 16 had hypersensitivity pneumonitis (HSP) and 14 had cryptogenic organizing pneumonia (COP/BOOP). Total BALF cells, cell density and cell differential counts were assessed and lymphocyte subsets CD3+, CD4+, CD8+, CD19+, CD3-CD16/56+ (NK) and CD3+CD16/56+ (NKT) were determined by flow cytometry.RESULTS: Significant differences were observed in the percentages of lymphocytes (p < 0.001) and CD3+CD16/56+ cells (p = 0.023) among patient groups. In patients with EP, the percentage of eosinophils correlated positively with the number of CD3-CD16/56+ cells (r = 0.522, p = 0.005), the percentage of CD3-CD16/56+ cells (r = 0.690, p < 0.001), and the absolute count of CD3+CD16/56+ absolute cells (r = 0.609, p = 0.001). However, in patients with IPF, HSP or COP/BOOP, no correlation between the percentage of eosinophils and CD3-CD16/56+ or CD3+CD16/56+ cells was observed.CONCLUSIONS: Eosinophil inflammation seems to develop through a different pathway in EP compared to other ILDs.