In order to study factors associated with changes in radiographic appearance and lung function after pleural effusion, we investigated 178 consecutive patients with non-malignant pleural effusion. At the initial examination etiology, smoking habits, asbestos exposure, ESR, blood eosinophils, size of effusion and other X-ray lesions were registered. At a 3-year follow-up, chest radiographs and lung function values were obtained and the association with the initially registered factors was evaluated. At follow-up, 20% of the patients had developed major additional X-ray lesions and/or significantly reduced lung function. Prognostically unfavourable factors were idiopathic etiology as compared to infectious, medium and large-size effusions and initial radiographs showing converging pleural linear structures and/or rounded atelectasis as compared to no or minor radiographic lesions. Converging pleural linear structures and rounded atelectasis were seen almost exclusively in association with idiopathic effusions. The obvious differences noted between patients with idiopathic and infectious effusions suggest that these effusions represent separate clinical entities.