T cells play an important role in the immune system and in the inflammatory response that determines the development and maintenance of psoriasis plaques. Better understanding of the pathophysiology of this disease has led to the development of specific biological treatments aimed at patients with extensive psoriasis. Traditionally, psoriasis has been treated with drugs which, in spite of their efficacy, have a toxicity associated to their long-term use. Thus, they cannot be used safely, comfortably or efficiently in many patients. Efalizumab, a biological agent specifically and selectively directed towards blocking the key steps in the pathogenesis of psoriasis, has been shown to be effective and safe in the short and long term in the treatment of psoriasis in more than 15 phase I, II and III clinical trials. In this article, the results of efficacy at 12 weeks, 6 months and three years are reviewed. Efalizumab arises as an important addition to the dermatological pharmacopoeia for the long-term treatment of psoriasis.