Background: Metastatic renal cancer is considered not to benefit from treatment by present-day methods. Clinical and experimental experience lead to the suggestion that mechanisms exerted by the immune system favour the outcome of patients with metastatic renal carcinoma. Immunomodulating therapy with interferon-gamma has been proved to be clinically effective in etastatic renal carcinoma. Material and Methods: A non-randomized phase-II study was initiated to observe the clinical and immunological effects of 200 μg interferon-gamma (IFN-gamma) s. c. once weekly in patients with metastatic renal carcinoma. Twenty patients were enrolled in this study. The protocol was accepted by the Ethical Committee of the Medical Faculty, University of Essen. Clinical data, cellular and humoral immune system parameters were monitored over 6 months. Results: Median survival of the patients was 12 months. No complete or partial remission could be documented. We observed a significant increase in lymphocyte, B, T, T helper and natural killer (NK) cell counts after 6 months of treatment compared to the appropriate pretreatment values. Concentrations of complement C3, C4 and immunoglobulins (Ig) A, IgG and IgM were also significantly increased after treatment with IFN-gamma. Pretreatment values of B, T, T helper and NK cells were decreased and complement serum values were increased in comparison to normal values. Conclusion: Although a significant increase of the investigated immunologie parameters after IFN-gamma therapy could be seen, no clinical remission was associated with this treatment modality. The validity and usefulness of immunomonitoring in metastatic renal cancer has to be discussed again.