The medical records of 507 patients with polymicrobial septicemia were examined to determine prognostic and descriptive factors. Over 50% of the episodes occurred in patients with solid tumors and 80% originated during hospitalization. Invasive procedures and immunosuppressive therapy frequently preceded development of polymicrobial septicemia, and infection was often accompanied by shock and pneumonia. A majority of infections were caused by at least 1 aerobic gram-negative bacillus (76%) and anaerobic infections were not infrequent. Overall response among these patients was 50%, with poorest response seen among patients with persistent neutropenia (25%), pneumonia (19%), and gram-negative bacillary infection (46%). Therapy with an antibiotic regimen to which all causative organisms were sensitive was of greatest prognostic significance. Response to appropriate therapy was 58%, whereas only 10% of those who received inappropriate therapy were cured (p less than .0001).