The term cryptogenic organising pneumonia has been used for the combination of dyspnoea, cough, pleuritic pain, widespread shadows on chest radiographs, and histological evidence of intra-alveolar organisation with buds of granulation tissue within the alveoli. We report 12 patients with seasonal recurrence of this disorder for between 3 and 11 years. In all 12 patients, symptoms recurred between late February and early May every year, tending to increase in severity each year, and resolved between June and January. Chest radiography and computed tomography showed bilateral consolidation. Lung biopsy samples showed intra-alveolar buds of granulation tissue. There were many neutrophils within the lumina of medium-sized airways and terminal bronchioles showed evidence of obstruction by granulation tissue. Functionally, the predominant defect was restrictive and only 2 patients (life-long non-smokers) had airflow limitation. All 12 patients had very high activities of liver enzymes, suggesting intrahepatic cholestasis, but no other evidence of liver disease. Cultures of blood, sputum, lung tissue, and bronchoalveolar lavage fluid, viral screening, and complement fixation tests were consistently negative. In all patients all abnormalities responded rapidly to oral steroid therapy. These findings suggest a seasonal syndrome of organising pneumonia and biochemical abnormalities indicative of intrahepatic cholestasis. No aetiological factor has been identified, but the nature and periodicity of the illness point to an inhaled agent present in the environment for a limited period every year.