Sexually transmitted genital ulcerative diseases are a worldwide health problem and have been strongly associated with an increased risk of human immunodeficiency virus (HIV) transmission. Chancroid, caused by the gram-negative pathogen Haemophilus ducreyi, is the leading cause of genital ulcer disease in developing countries of Asia and Africa. In Europe and North America, H. ducreyi infections account for a smaller proportion of reported genital ulcers. However, recent studies in the United States suggest that many more cases of chancroid occur than are reported. Haemophilus ducreyi has been the focus of recent research aimed at understanding the mechanisms of pathogenesis of chancroid and its association with HIV transmission.