The study of pharmacogenetics will optimize the use of systemic therapies in dermatology. Directed prescription of azathioprine dependent on phenotypic expression of the thiopurine methyltransferase gene is now accepted practice. To some extent other, older drugs have been neglected. We look at the role that pharmacogenetics could play in the use of methotrexate for psoriasis, focusing on known polymorphisms in the folate metabolic pathway.