Several trials have evaluated the therapeutic efficacy of rIL-2 combined with more traditional treatments such as chemotherapy and radiotherapy, but the use of IL-2 as adjuvant therapy for minimal residual disease or to maintain clinical response obtained with other standard treatments has yet to be investigated. The aim of the present trial was to study the biological effects of maintenance long-term treatment (6 months) with subcutaneous low-dose IL-2 in 16 patients with different neoplasms previously treated with chemo-immuno therapeutic regimens or with surgery (7 metastatic renal cancers, 5 locally advanced renal cancers previously subjected to radical nephrectomy, 2 metastatic breast cancers, 1 small cell lung cancer, and 1 metastatic melanoma). Clinical tolerability, feasibility and therapeutic implications are also discussed. The IL-2 schedule was as follows: 4.5 million IU/day, 3 times weekly for 6 months. A total of 14 patients completed therapy without requiring dose modifications and are free of progression after a median duration of 8+ months (range: 7+ to 34+) while two patients progressed during therapy (one inflammatory breast cancer and one renal cancer). Important and persistent hemato-immunostimulating effects in both soluble (IL-2, sIL-2R, IL-6) and cellular (lymphocyte subsets, monocytes, eosinophils) parameters were noted during the entire treatment. The IL-2 related toxicity was quite low. Moreover, this long-term IL-2 therapy could control neoplastic growth and thus prolong clinical response obtained with standard treatments. Prospective randomized studies regarding the clinical efficacy have been initiated.