Gram-negative sepsis, which was uncommon in the pre-antibiotic era, has become a major problem throughout the latter half of this century, particularly among persons with severe underlying diseases. The development of antibiotics with dramatic bactericidal activity against these pathogens and advances in supportive care have improved the outlook. Nonetheless, the mortality remains high among the growing numbers of elderly patients with chronic illnesses, immunocompromised patients, patients subjected to invasive procedures, and patients in critical care units colonized with highly antibiotic-resistant flora.