As several studies have demonstrated a relationship between decreased serum selenium concentrations and the frequency of certain cancers, we studied these concentrations in two kinds of cutaneous tumour cancer: melanoma and epidermotropic cutaneous lymphoma. We first determined the predictive value of the selenium assay for the frequency of recurrences in stage I and II melanomas and then considered the relationship between serum selenium concentrations before treatment and therapeutic response. Two hundred melanomas (81 stage I, 63 stage II, 56 stage III) and 51 epidermotropic cutaneous T-cell lymphomas (CTCL) (8 stage I, 24 stage II, 10 stage III, 9 stage IV) were included in the study. Selenium assays were performed by atomic absorption spectrophotometry (92 +/- 16 micrograms/l in 30 normal subjects). Our study showed decreased serum selenium concentrations for melanoma (81 +/- 27 micrograms/l) and lymphoma (78 +/- 36 micrograms/l) relative to disease severity. The concentration was significantly lower (76 +/- 22) for stage I and II melanomas with recurrence within 2 years (31 patients), compared to those without recurrence (113 patients) (p < 0.05). Before treatment, it was higher in CTCL with good response to treatment (89 +/- 36) than in those without response (62 +/- 30) (p < 0.01). This study thus demonstrates the prognostic value of selenium assays in the follow-up of melanoma and CTCL.