Idiopathic Pulmonary Fibrosis/ Usual Interstitial Pneumonia (IPF/UIP) represents the most important interstitial pneumonia. ATh2 cytokine pathway predominance, favoring collagen deposition, associated to a deficit in (IFN- gamma) network, seems to be involved in the pathogenesis of this disease. Nonetheless, few data are available about the potentially involved cells. Natural killer cells (NK), are one of the most important subsets implicated in the IFN-gamma network. The aim of this study was to assess NK cells, both in BAL and peripheral blood of 11 patients suffering from IPF (group A) with respect to 11 patients with other interstitial pneumonia (Group B). Our results did not show any statistically significant difference in NK percentage in BAL between group A and B. On the contrary, patients with IPF showed a higher percentage of NK cells (t = 2.41; p < 0.05) and absolute number of cells (t = 2.32; p < 0.05) in peripheral blood, as well as a strong positive correlation between circulating and BAL NK cells (r = 0.69; p < 0.05). This finding shows, for the first time, a relationship between peripheral and lung resident cell environments in humans suggesting a possible systemic involvement in the natural history of IPF.