Systemic mast cell disease is a rare disease of unknown aetiology. Systemic infiltration and proliferation of mast cells in skin, bone marrow, gastrointestinum and lymph nodes is the central pathological feature. This study reports a patient with mastocytosis of the skin (urticaria pigmentosa) for 10 yrs. The patient was referred to hospital for dyspnoea. Chest radiograph showed moderate reticular infiltration of both lungs, computerized tomography revealed multiple lymph nodes of the mediastinum and faint nodular lesions of middle and upper areas of lungs. Transbronchial biopsy demonstrated mast cell infiltration of the lung with formation of mast cell granuloma. According to the current literature, systemic mast cell disease with pulmonary involvement is a very rare entity. After a treatment with interferon a2a over 6 months, the patient's condition and particularly dyspnoea showed improvement in parallel with an amelioration of the lesions as demonstrated by thorax computed tomography. Eur Respir J 2000; 15: 623±625. Mastocytosis is a local or systemic disease characterized by infiltration and expansion of mast cells in various tissues. Urticaria pigmentosa is diagnosed when skin is affected systemically showing multiple brown or livid papules.Whenever mast cell infiltration involves additional organs, the diagnosis of systemic mast cell disease (SMCD) is applied. Involvement of the gastrointestinal tract and bone marrow is most common. The patient presented here showed systemic mastocytosis with a biopsy-proven mast cell infiltration of the bone marrow and lung. Case reportThe present 54-yr-old male with a moderate smoking history had chronic pruritus for 10 yrs. Integument revealed exanthematic red brown papules with major involvement of the truncus. Rubbing of the skin evoked urticarial swelling. A cutaneous skin biopsy showed keratinocytes with increased pigmentation and proliferation of melanocytes. In the upper dermis, perivascular infiltrates consisted of lymphocytes, histiocytes and huge numbers of mast cells, all alterations indicative for urticaria pigmentosa. A total of 24 mast cells per high power field (magnification 6400) were counted. Figure 1 shows the truncal skin of the patient with the typical dermal manifestation of mast cell disease. The patient then presented with dyspnoea. The chest radiograph showed signs of interstitial lung disease with a fine reticular infiltration of both lungs ( fig. 2A). Computed tomography (CT) of the thorax revealed faint nodular and cystic lesions of the middle and upper fields of the lungs with mediastinal adenopathy (fig. 2B). Blood gas analysis at rest gave a mild hypoxaemia with an arterial oxygentension (Pa,O 2 ) of 8.5 kPa (64 mmHg). Pulmonary function tests showed a modest obstruction with a forced expiratory volume in one second (FEV1) of 3.05 L (77% of predicted value) and a normal vital capacity (VC) of 4.82 L (92% pred). Carbon monoxide transfer factor (TL,CO) was markedly diminished (55% pred). Laboratory chemical examinations yielded a p...