Immunotherapy with BCG is effective in patients with recurrent superficial bladder carcinoma. This therapy involves Interleukin-2 (IL-2), but little is known about the immunological parameters involved in superficial bladder carcinoma. We have monitored immunological parameters in twenty patients with superficial bladder carcinoma treated with transurethral resection (TUR) followed by IL-2 instillation. Cell numbers of peripheral blood leukocyte subpopulations were counted before surgery and during follow-up after surgery. During follow-up, we compared the cell counts in patients with and without a recurrent tumour. We used the values of healthy matched controls as a reference. Recurrent disease in patients corresponded with a significant increase in CD8+ lymphocytes, and especially the CD8highCD57+ and CD8low subpopulations. The phenotype of these T lymphocytes belongs to cells with an immunosuppressive function. We hypothesize that these peripheral immune suppressive cells facilitate tumour recurrences or that tumour recurrences cause an increase in peripheral immune suppressive lymphocytes.