Interferon α is a cytokine currently approved worldwide for a number of antiinfective and antiproferative indications. It is administered at least several times per week because of its rapid clearance from the body. To reduce the frequency of administration, and potentially to increase efficacy interferon α molecules are modified by pegylation. Pegylation is a process, in which one or more chains of polyethylene glycol are attached to the interferon a molecule, providing a selectively protective barrier from rapid absorption, metabolism and elimination. The efficacy of pegylated interferon-α2a and interferon-α2b was evaluated for the treatment hepatitis C and B, chronic myelogenous leukemia and metastatic renal cell carcinoma. Data from clinical trials indicated that pegylated interferon α has distinct pharmacokinetic advantages compared to interferon α and a tolerable side effect profile. The efficacy and toxicity of pegylated interferon α will be further assessed in clinical trials and compared with interferon α.