To assess the usefulness of the S-100B tumour marker in monitoring therapy and during follow-up we measured S-100B serum concentration in 31 patients participating in a phase I trial studying the effect of vaccination with GM-CSF gene-transduced autologous tumour cells. The S-100B serum concentration before treatment is a strong independent prognostic factor for survival, as in this study the 11 patients with low (< 0.16 microgram/l) S-100B levels were at considerably lower risk for death. All patients alive at the end of the study (n = 8) had a S-100B level within the reference range at the start of the study. At a level of 0.16 microgram/l the presence of disease could be predicted with 99% certainty for 63% of the patients. Nine patients became progressive after a stable disease. In four of them S-100 increased over 50% before clinical observation of progression, giving a lead time of more than a month in 44% of cases. In patients who acquired a status of no clinical evidence of disease after treatment, the S-100B concentration invariably decreased by more than 100%.