After the initial dramatic effects, observed in a Lewis lung carcinoma animal model, using a combination of thymosin alpha 1 (Tα1) and interferon (IFN) after cyclophosphamide, a number of other preclinical models in mice (Friend erythroleukemia and B16 melanoma) and in rats (DHD/K12 colorectal cancer liver metastasis) have confirmed the efficacy of the combination therapy with Tα1 and either IFN or IL-2 plus chemotherapy. These results provided the scientific foundation for the first clinical trials using Tα1 in combination with BRMs and/or chemotherapy. Pivotal trials in advanced non-small cell lung cancer (NSCLC) and melanoma with Tα1 and IFN-α low doses after cis-platinum or dacarbazine produced the first evidence of the high potentiality of this approach in the treatment of human cancer. The combination of Tα1 and IFN-α was also used in patients affected by chronic B and C hepatitis including IFN-nonresponders and infected by precore mutants or genotype 1b. Further studies demonstrated additional biological activities clarifying the mechanism of action of Tα1, partially explaining the synergism with IFN. It has been shown the capacity of activating infected dendritic cells through Toll-like receptor signaling, thus influencing the inflammation balance, and of increasing the expression of tumor, viral, and major histocompatibility complex (MHC) I antigens. Dose-response studies suggested the possibility of improving the efficacy of this molecule reducing the overall toxic. Based on these information two clinical trials are ongoing: a large phase II on advanced melanoma patients treated with Tα1 at different doses after dacarbazine and a phase III one, on IFN-resistant hepatitis C virus (HCV) patients treated with a triple combination (IFN, ribavirin, and Tα1).