Maintenance chemotherapy provides only a modest survival advantage in metastatic breast cancer (MBC). We have previously shown that a maintenance immunotherapy (MI) regimen based on low-dose interleukin-2 (IL-2) and 13-cis retinoic acid (RA) improved the lymphocyte and natural killer cell (NK) counts, and CD4+/CD8+ ratio in patients with a clinical benefit from chemotherapy. With the aim of improving progression-free survival (PFS), 100 consecutive MBC patients with a clinical benefit from chemotherapy were treated with an MI. Patients with MBC were eligible if they had no evidence of progression after 6-8 courses of epirubicin-paclitaxel induction chemotherapy. Treatment consisted of low-dose IL-2 and oral RA given until progression. The primary endpoint was progression-free survival (PFS); secondary endpoints were toxicity, overall survival (OS), and changes in immunological parameters. From 04/1997 to 04/2002, 100 patients with MBC were enrolled. After a median follow-up of 49 months, median PFS and OS were 37.1 and 57.5 months, respectively. No WHO grade 3 or 4 toxicity was observed; grade 2 cutaneous toxicity and autoimmune reactions occurred in 19 and 16% of patients, respectively. A sustained improvement in lymphocytes, NKs, and in the CD4+/CD8+ ratio was observed, with respect to baseline values. In conclusion, MI with IL-2 and RA in MBC patients who do not progress after 6-8 courses of chemotherapy is well-tolerated, improves lymphocyte, NK, CD4+/CD8+ ratio, and appears to delay disease recurrence. A randomized trial is warranted.