Regarded for nearly 80 years as a rare disease, melioidosis is emerging as an important cause of morbidity and mortality in North East Thailand, and is probably underdiagnosed elsewhere. Infection with environmental Pseudomonas pseudomallei occurs during the rainy season, but the mode of acquisition is usually unclear, and the infection may remain latent for several years. Much remains to be learned about the interactions between host defence mechanisms and bacterial virulence factors in the disease. Fulminant septicaemia in immunocompromised patients is the most frequent clinical manifestation, although serological surveys suggest that self-limiting infections are widespread in the normal population. Since diagnosis still depends on isolation of the causative organism, rapid and improved serodiagnostic tests are urgently needed. New beta-lactam drugs, such as ceftazidime, have significantly reduced the mortality of the disease, although the emergence of resistance and late relapses of infection present challenging therapeutic problems.