Intravesical instillation of Bacillus Calmette-Guérin (BCG) offers safe and effective short-term/long-term treatment for superficial transitional cell carcinoma (TCC) and TCC in situ of the bladder. However, 17 to 42% of patients may experience recurrence in spite of this therapy and a marker of effective treatment is of paramount importance. In this study the in vitro response of peripheral blood lymphocytes (PBL) to BCG was analysed in 10 patients with superficial TCC and TCC in situ before and during BCG instillations. The in vitro response of PBL to BCG, expressed as a stimulatory index (SI), revealed that 6 patients had a SI > 5 and 4 patients had a SI < 5. None of the former patients had recurrence of TCC during a mean follow-up of 17 months, while all of the latter patients experienced recurrence of TCC within 9 months. It was concluded that the in vitro response of PBL to BCG may be used as a marker of response to intravesical BCG treatment.