Many things have changed since Hamman and Rich's first description of fulminant "diffuse interstitial lung fibrosis" in the 1930's. The most recent classification of Katzenstein follows the logic of Leibow, a logic that has been ignored for decades. The main differentiation is the separation from usual interstitial pneumonitis (UIP) of a specific disease entity with similar clinical characteristics "non specific interstitial pneumonitis" (NSIP). This entity has also characteristic CT appearance and most importantly much better prognosis since it responds to treatment with corticosteroids. The new classification has changed our diagnostic and therapeutic strategy for the diffuse interstitial lung diseases. Today we believe that: i) idiopathic pulmonary fibrosis (IPF) is a specific disease entity with the histologic pattern of UIP, ii) the separation of NSIP from UIP most probably means that old "benign" cases of IPF where in fact NSIP and this is why they had better clinical course, iii) the diagnosis of IPF can be obtained without lung biopsy if major clinical criteria are met and high resolution CT has minimal ground glass appearance. Whenever clinical/radiologic picture is not clear, open lung biopsy is imperative in order to rule out entities with better prognosis, mainly NSIP, iv) The suggested therapeutic regimen is combination of corticosteroids with azathioprine or cyclophosphamide and v) if we are going to treat we must do so as early as possible. Pneumon 2001, 14(3):174-183