We report the prognostic importance of strength of reaction to BCG, tumor histology and clinical factors in patients with previously untreated high-risk (Clark, III, IV and V) primary malignant melanoma. One hundred and one such patients receive high-dose BCG (1 x 10(8) viable units) by Heaf gun as an adjuvant to standardized primary surgery according to EORTC Protocol 18741. Univariate analysis of disease-free interval (DFI) indicates that the degree of maximum reaction to BCG (p = 0.0003), Breslow thickness (p = 0.0003) and Clark level (p = 0.002) are highly significant prognostic factors. When a multivariate model using Cox's proportional hazard regression was used for DFI, the degree of maximum reaction to BCG and Breslow thickness were by far the most significant criteria. A prognostic equation wa obtained to predict DFI from maximum BCG reaction and Breslow thickness. From analysis of the "scores" calculated in this way it appears that the two variables act independently. This technique permits the determination of values that are predictive of DFI and discriminate between subgroups of patients with different DFI characteristics (5 groups, p less than 0.0001). This exercise was repeated for survival and similar results were obtained. The degree of a patient's immune reaction to BCG administered therapeutically is of paramount importance in determining the likelihood of survival. This factor and the Breslow thickness can be integrated to produce a mathematical equation which accurately predicts survival for appropriately treated melanoma patients.