Aldesleukin [recombinant interleukin-2 (IL-2)] is a biological response modifier which, like endogenous IL-2, has a range of immunomodulatory properties. Although aldesleukin is currently approved only for treatment of renal cell carcinoma, it has been widely used in patients with metastatic melanoma and has been assessed in numerous noncomparative trials in this indication. Durable complete responses have been reported in a small proportion of patients with good performance status who received immunotherapy or chemoimmunotherapy involving aldesleukin in several clinical trials. Overall median survival times of about 10 or 11 months are typical. Combination chemoimmunotherapy involving aldesleukin has produced relatively high response rates (up to 56%), but these are not predictive of increased survival time. Aldesleukin has not been directly compared with standard chemotherapy in randomised studies. The use of continuous intravenous infusion and subcutaneous administration of aldesleukin, together with improved patient selection, has successfully reduced the severity of adverse events produced by the drug (the original intravenous bolus regimen is particularly toxic and often necessitates admission to an intensive care unit). However, the risk/benefit profile of aldesleukin in the treatment of melanoma requires further study, particularly after subcutaneous administration. Thus, aldesleukin has shown modest efficacy in the treatment of metastatic melanoma. Although aldesleukin-containing regimens have produced promising results in many noncomparative trials, their clinical value in comparison with standard chemotherapy remains to be determined in phase III studies. Until results from such trials are available, clinicians will need to carefully balance the potential benefits of the drug against the risks and likely quality-of-life implications associated with its toxicity in each patient. Current investigations centre on the use of aldesleukin as part of complex biochemotherapy regimens, and results from these trials are awaited with interest.