We sought to review the definition, diagnosis, prognosis, and treatment of high-grade Ta urothelioma carcinoma and carcinomas in situ (CIS) in order to provide evidence-based guidelines for their diagnosis and treatment. The English-language literature on high-grade Ta urothelial carcinoma and CIS was identified and critically reviewed by a panel of 9 international experts. The panel then met at a consensus conference to present their conclusions. Levels of evidence and grades of recommendation were assessed. Findings from approximately 100 publications appearing prior to February 2005 were reviewed and summarized. High-grade Ta urothelial carcinoma and CIS are relatively rare tumors; thus results are often based on small nonrandomized studies. Their assessment is made more difficult owing to inaccuracies in staging and grading. Although there were similar numbers of level 1, level 2, and level 3 evidence citations, guidelines have been developed based only on levels of evidence supporting grade A and grade B recommendations. These evidence-based guidelines have been developed to aid clinicians in the diagnosis and treatment of patients with high-grade Ta urothelial carcinoma and CIS.