This article discusses the development of international guidelines for the diagnosis, treatment, follow-up, and prevention of low-grade Ta urothelial carcinoma of the bladder. The authors, who are experts in this field from 3 continents and 7 countries, reviewed the English language literature through September 2004. The results of the authors' deliberations are presented here as a consensus document. The objective of this study was to determine the optimal diagnostic workup, treatment, follow-up, and prevention of low-grade, Ta urothelial carcinoma of the bladder. A consensus conference convened by the World Health Organization (WHO) and the Societe Internationale d'Urologie (SIU) met to critically review the literature on the diagnosis and treatment of low-grade Ta urothelial carcinoma of the bladder. Research was conducted using Medline; this search engine also was used to identify additional works not detected at the initial search. Evidence-based recommendations for diagnosis and management of the disease were made with reference to a 4-point scale. Low-grade Ta urothelial carcinoma of the bladder is a well-studied subject with many level I and 2 evidence references that support clinical practice. Findings from 135 reviewed citations are summarized. Many grade A and B recommendations on the diagnostic workup and management of this disease can be given with level 1 and 2 evidence based on prospective randomized clinical trials of sufficient statistical power. This should improve the quality of the treatment of this disease.