Chancroid is a sexually transmitted disease characterized by genital ulceration. Although the pathology of chancroid lesions has been well described, we still know relatively little about the mechanism by which the causative agent, the bacterium Haemophilus ducreyi, produces disease. It is hoped that a better understanding of both the pathogenicity of H. ducreyi and the pathogenesis of chancroid will provide a rational basis for the development of improved strategies for the control of this disease.