This study represents a retrospective analysis of 270 patients with gram-negative bacteremia and demonstrates the importance of underlying disease as a major determinant of fatality. A significantly lower fatality ratio was found only among patients who received appropriate as contrasted with inappropriate antimicrobial treatment in the nonfatal category. No significant differences in fatality ratios were found among patients in comparable categories of underlying disease whose infections were hospital or nonhospital acquired or who had received prior treatment with adrenal corticosteroids, immunosuppressive agents, or antimicrobials. The only sequential yearly changes in prevalence of individual organisms were a higher proportion of Proteus sp during the first three years and of Klebsiella-Aerobacter during the last three years. There was neither a yearly trend toward increased frequency of resistance to two or more antimicrobials nor a significant difference between hospital and nonhospital acquired strains.