The therapeutic efficacy of biochemotherapy in metastatic malignant melanoma still carries a low remission rate, but with some durable responses. It would therefore be of considerable importance if patients with a high probability of responding could be identified using predictive tests. The response to interferon-alpha (IFN-alpha) correlates with the occurrence of CD4(+) lymphocytes identified by fine-needle aspirates from melanoma metastases (Håkansson et al, 1996). The present investigation studies a possible correlation between tumour-infiltrating CD4(+) lymphocytes in malignant melanoma metastases and the therapeutic effect of biochemotherapy. A total of 25 patients with systemic and 16 with regional metastatic melanoma were analysed before initiation of biochemotherapy (cis-platinum 30 mg/m(2) d.1-3, DTIC 250 mg/m(2) d.1-3 i.v. and IFN-alpha 2 b 10 million IU s.c. 3 days a week, q. 28d.). A monoclonal antibody, anti-CD4, was used to identify tumour-infiltrating lymphocytes in fine-needle aspirates before start of treatment. The presence of these lymphocytes was correlated to response, time to progression and overall survival. A statistically significant correlation (P = 0.01) was found between the occurrence of CD4(+) lymphocytes and tumour regression during biochemotherapy in patients with systemic disease. Out of 14 patients with moderate to high numbers of infiltrating CD4(+) lymphocytes, 12 achieved tumour regression. In contrast, among patients with low numbers of these cells in metastatic lesions, 8 out of 11 had progressive disease. We also found a significantly longer time to progression (P < 0.003) and overall survival (P < 0.01) among patients with moderate to high numbers of these cells compared to patients with low numbers of these cells before initiation of biochemotherapy. Furthermore, in patients with regional disease, we found a significantly longer time to progression (P = 0.01) and a trend toward a longer overall survival time (P = 0.09). Based on these results and as previously shown with IFN-alpha therapy alone, there seems to be a need for CD4(+) lymphocytes infiltrating the tumours before the start of biochemotherapy to make the treatment successful. Determination of these cells in fine-needle aspirates seems to be a method to predict responders to biochemotherapy, thus increasing the cost-benefit of this treatment strategy considerably, both in terms of patient adverse reactions and health care costs.