The National Cancer Institute Bladder Tumor Marker Network conducted a study to evaluate the reproducibility of immunohistochemistry for measuring p53 expression in bladder tumors. Fifty paraffin blocks (10 from each of the five network institutions) were chosen at random from among high-grade invasive primary bladder tumors. Two sections from each block were sent to each laboratory for staining and scoring, and then all sections were randomly redistributed among the laboratories for a second scoring. Intra- and interlaboratory reproducibility was assessed with regard to both staining and scoring. For overall assessments of p53 positivity, the results demonstrated that intralaboratory reproducibility was quite good. Concordance across the five participating laboratories was high for specimens exhibiting no or minimal nuclear immunostaining of tumor cells or high percentages of tumor cells with nuclear immunoreactivities. However, there was a reduced level of concordance on specimens with percentages of stained tumor cells in an intermediate range. The discordancies were due mainly to staining differences in one of the five laboratories and scoring differences in another laboratory. These results indicate that some caution must be used in comparing results across studies from different groups. Standardization of staining protocols and selection of a uniform threshold for binary interpretation of results may improve assay reproducibility between laboratories.