Summary— The local immune response to intravesical BCG therapy for widespread carcinoma in situ and superficial bladder tumour was assessed using immunohistochemical methods on serial biopsies obtained during treatment. The main findings were the induction of a strong level of HLA–DR expression by urothelial cells which persisted for several months after completion of therapy. This coincided with a mononuclear cell infiltrate in the bladder wall consisting predominantly of activated helper T lymphocytes. Smaller numbers of macrophages, suppressor cytotoxic T lymphocytes and B lymphocytes were also found. These new findings confirm that the inflammatory response to BCG involves the urothelium in an immunological reaction and suggest that the success of immunotherapy with intravesical BCG may be due to enhanced tumour antigen recognition.