A 48-yr-old nonsmoking female presented with a 5-yr history of recurrent bronchitis with progressive exercise dyspnoea. She had been diagnosed with a small monoclonal gammopathy of undermined significance (MGUS) 6 yrs previously. Inspiratory vital capacity was 4.7 L (125% predicted) and forced expiratory volume in one second/forced vital capacity (FVC) ratio was 79%. Carbon monoxide transfer factor was markedly impaired (35% pred), and arterial oxygen tension ( P a,O2) decreased from 9.2 to 6.0 kPa at exercise (40 W, 10 min). A thoracic computed tomography (CT) scan was performed (fig. 1⇓). Fibreoptic bronchoscopy found inflammation of the bronchial mucosa with candle stain-like deposits on the trachea and large bronchi, without any purulent secretion within the respiratory tract (fig. 2⇓). The histopathological aspect of bronchial biopsies is shown in figure 3⇓. Blood tests disclosed type I cryoglobulinaemia composed of a 32 g·L−1 monoclonal immunoglobulin (Ig)Mκ serum component (at 37°C). Blood cell count was normal. Bone marrow examination showed malignant lymphoplasmacytic cells representing 30% of total cells, with a monotypic surface expression of IgMκ. The C-reactive protein level was 21 mg·L−1. Extensive renal, cardiac and hepatic evaluations were normal. As severe hypoxaemia and reduction of carbon monoxide transfer factor were unexplained, the patient underwent videothoracoscopic lung biopsies. Fig. 1— Thoracic computed tomography scan. Fig. 2— Fibreoptic bronchoscopy. Arrow shows candle stain-like deposits. Fig. 3— a) Bronchial biopsy of candle-stain like lesions stained by haematoxylin-eosin-saffron. Submucosal abnormal deposits with concentric pattern around the vessels (black arrow), arranging in cords or ribbon-like structures in the interstitium (black dashed arrow) or in an organoid, raspberry-like fashion in smooth muscle fascicles (black arrowhead). b) Bronchial biopsy; perivascular deposits with negative Congo red dye staining. c) Direct anti-κ-light chain immunofluorescence in bronchial biopsy showing positivity of perivascular (white arrow) and interstitial (white dashed arrow) deposits, and epithelial …