Initial reports focused on purified protein derivative skin test reactivity and granuloma formation in patients treated with intravesical bacillus Calmette-Guérin. Then some investigators using immunohistochemical methods showed that some lymphocyte phenotypes where predominant in such infiltrates and that HLA DR antigens where strongly expressed following BCG therapy. Similar results where obtained in urine using flow cytometric analysis. Additionally some cytokines where found at significant levels in urine after BCG instillations. In this paper we reviewed these studies, we discussed the meaning of these results and the potential value for treatment monitoring.