We have observed that an early increase in the CD4+/CD8+ ratio of metastatic melanoma patients during chemoimmunotherapy is the most favourable independent prognostic factor. In this study, 87 patients with metastatic melanoma were monitored for peripheral blood lymphocyte subsets (CD4+ and CD8+) before and during chemoimmunotherapy (dacarbazine, vinblastine, lomustine and bleomycin or dacarbazine alone plus interferon-alpha) to confirm our previous observation. Blood samples were systematically obtained from patients who received either of these chemoimmunotherapies. The frequencies of peripheral blood lymphocyte subsets were monitored by flow cytometry using monoclonal antibodies OKT4 (CD4+, T-helper cells) and OKT8 (CD8+, T-suppressor cells). The overall response rate was 46.5%, and the median overall survival was 9.3 months. Patients with pre-treatment CD4+ levels above the mean level, who later responded to therapy, had a median survival of 28.1 months vs. 10.2 months for responders with low pre-treatment CD4+ levels (P=0.053, log-rank test). Responders with decreasing CD8+ levels had a median survival of 27.8 months vs. 10.8 months for responders with increasing CD8+ levels (P=0.04, log-rank test). Similarly, responding patients with an increasing CD4+/CD8+ ratio had a median survival of 28.1 months vs. 10.5 months for those with decreasing ratios (P=0.002, log-rank test). Non-responders showed no difference in median survival irrespective of low or high pre-treatment CD4+ lymphocytes, increasing or decreasing CD8+ levels, or increasing or decreasing CD4+/CD8+ ratios. This follow-up study confirms that the monitoring of CD4+ and CD8+ lymphocytes may provide important predictive and prognostic information in metastatic melanoma patients receiving chemoimmunotherapy.