The aim of this study was to investigate the effect on lung function of lung biopsy used in the diagnosis of diffuse lung disease carried out by an open procedure or by video-assisted thoracoscopy. One hundred and sixteen patients with diffuse lung disease who attended the Royal Brompton Hospital were studied retrospectively. Thirty five patients underwent open lung biopsy, and 33 video-assisted thoracoscopic biopsy and 48 had their diagnosis made without biopsy. All patients underwent lung function tests before and after surgery, or at an interval of 3-6 months in those who did not undergo biopsy. No significant differences were found in changes in lung function between those who had and had not undergone biopsy, and the proportions of patients whose lung function improved or deteriorated were similar. Lung biopsy by an open procedure or by video-assisted thoracoscopy did not differ in its effects on lung function. The results for older patients, those with severe disease and those with fibrosing alveolitis were the same as for the whole group. Open lung biopsy for the diagnosis of diffuse lung disease does not deleteriously affect lung function whether carried out by an open or a minimally invasive procedure.