Importance of the field: The advent of biologic therapies has revolutionized the treatment of autoimmune diseases including psoriasis, autoimmune arthritides and inflammatory bowel disease. With recent advances in our understanding of the immunogenetic pathways involved in the pathogenesis of these conditions, newer, more targeted biologic therapies have been developed. Ustekinumab is an antibody to the common p40 subunit of IL-12 and IL-23, which has been studied in the treatment of psoriasis, psoriatic arthritis, Crohn's disease and multiple sclerosis. Areas covered in this review: This review details the efficacy and safety of ustekinumab in all clinical studies to date, using PubMed listed publications and official product websites. What the reader will gain: Readers will gain a comprehensive understanding of the mechanism of action of ustekinuamb, its pharmacodynamic and pharmacokinetic profile, and its clinical efficacy and safety in the treatment of psoriasis, psoriatic arthritis, Crohn's disease and multiple sclerosis. Take home message: Ustekinumab has shown significant efficacy in the treatment of chronic plaque psoriasis in Phase III studies, and promising results in Phase II studies in psoriatic arthritis. Efficacy has been shown in Crohn's disease only in non-responders to infliximab. Ustekinumab did not show benefit in the treatment of multiple sclerosis. © 2010 Informa UK Ltd.