Syphilis is an important sexually transmitted infection. There has been an epidemic of syphilis amongst men who have sex with men in the United Kingdom in the last decade. Early infection with syphilis causes a genital ulcer followed, in the absence of treatment, by a generalised illness often accompanied by rash. Studies on the pathogenesis of syphilis have been limited because it is not possible to grow syphilis outside of the body. New approaches using molecular tests allow the immune response of the patient to infection to be measured directly from a swab of a genital ulcer or rash and/or a blood sample and also allow the whole genetic sequence of the bacteria to be obtained from a swab.