BACKGROUND Since the immune system is considered to be a major determinant in the outcome of malignant melanoma, vaccination with BCG (bacillus Calmette-Guérin) or more recently with interleukins is used in this condition, although the effects of this immunotherapy are unclear. OBJECTIVE The present study was to elucidate clinical aspects and the prognostic relevance of the cellular and humoral immune responses of melanoma patients treated with two different BCG vaccines. METHODS A subgroup of patients from a multicenter adjuvant trial with BCG in stage I (pT3-4N0M0) high-risk melanoma was prospectively subjected to detailed analysis regarding tuberculin (PPD, purified protein derivate) skin test reactivity, local, regional and systemic reactions to BCG vaccination, PPD antibody response and disease-free survival. Patients were randomized into three arms and received either no adjuvant treatment (22 patients), BCG RIV (40 patients) or BCG Pasteur (44 patients). All patients were followed for up to 10 years (median follow-up 6 years). RESULTS Patients treated with BCG Pasteur mounted a stronger antibody response, experienced stronger regional and systemic reactions to vaccination and converted more frequently to positive PPD skin tests, compared to controls and to patients vaccinated with BCG RIV. All BCG-treated patients who developed an antibody response had a longer disease-free interval (p = 0.05), with slightly higher significance for BCG Pasteur-treated patients (p = 0.02). CONCLUSIONS Cellular as well as humoral immune responses to PPD and BCG thus identify stage I malignant melanoma patients with an overall better prognosis.