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LIP was also described as early as 1969 by Liebow. For many years, the significance was low in view of the inability to differentiate it from pulmonary lymphomas (mucosa-associated lymphoid tissue [MALT] lymphomas). It was only with the newer molecular biology and immunohistochemical methods that it was possible to reliably differentiate malignant from reactive changes. Pure idiopathic LIP is certainly a very rare disease, occasionally it occurs in conjunction with underlying systemic diseases such as rheumatoid arthritis, Sjögren’s syndrome, pernicious anaemia, chronic active hepatitis, SLE, primary biliary cirrhosis, myasthenia gravis, severe immune deficiency syndromes (AIDS) and others. The disease more frequently affects women in the fifth decade of life and has a slow progressive course with dry cough and exertional dyspnoea. There is rarely a marked fibrosing reaction, but instead extensive, diffusely distributed infiltration of the alveolar septa with lymphocytes, plasma cells and histiocytes, often also with the hyperplasia of MALT. However, a distribution along the lymphatics (bronchovascular bundle, pleura, interlobular septa) should suggest lymphoma. The pneumocytes are hyperplastic and intra-alveolar organisation tissue is also occasionally present. The HRCT is marked by a ground glass appearance and reticular markings, and occasionally perivascular cysts or honeycombing.