On 9-10 September 1999, an international workshop on image analysis and quantification in lung tissue was held at the Leiden University Medical Center, Leiden, The Netherlands. Participants with expertise in pulmonary and/or pathology research discussed the validity and applicability of techniques used for quantitative examination of inflammatory cell patterns and gene expression in bronchial or parenchymal tissue in studies focusing on asthma and chronic obstructive pulmonary disease (COPD). Differences in techniques for tissue sampling and processing, immunohistochemistry, cell counting and densitometry are hampering the comparison of data between various laboratories. The main goals of the workshop were to make an inventory of the techniques that are currently available for each of these aspects, and in particular to address the validity and unresolved problems of using digital image analysis (DIA) as opposed to manual scoring methods for cell counting and assessment of gene and protein expression. Obviously, tissue sampling and handling, fixation and (immunohistochemical) staining, and microscope settings, are having a large impact on any quantitative analysis. In addition, careful choices will have to be made of the commercially available optical and recording systems as well as the application software in order to optimize quantitative DIA. Finally, it appears to be of equal importance to reach consensus on which histological areas are to be analysed. The current proceedings highlight recent advances and state of the art knowledge on digital image analysis for lung tissue, and summarize the established issues and remaining questions raised during the course of the workshop.