Obstructive lung disease in two patients with Behcet's syndrome is presented. In both cases increased airways resistance, impaired PEF and FEV1.0 were found. In addition, diffusion capacity was slightly decreased in spite of enlarged total lung capacity and increased residual volumes. Both patients had mild hypoxaemia despite slight hyperventilation; a normal chest X-ray, and a decreased serum complement C3 level. Corticosteroids had a beneficial effect on VC in one of the two cases but failed to have any significant effect on the impaired ventilatory function in the other.