Between 1971 and 1987, 97 patients with polymicrobial bacteremia (PMB) were seen by a consulting infectious disease service. Seventy-four had severe underlying illnesses, and infection was hospital acquired in 80. PMB resulted from intraabdominal, urinary tract, or soft tissue infection in 45 patients, but a wide range of sources were implicated in the rest. Eleven patients had more than one source for the bacteremia, and, despite intensive diagnostic efforts, 24 had no identifiable source for at least one blood isolate. Bacteremia due to gram-negative bacilli most commonly occurred in intraabdominal, urinary tract, and wound infections; Escherichia coli and Klebsiella species were most frequently isolated. Streptococcus faecalis and Staphylococcus aureus were the predominant gram-positive isolates. Certain bacterial combinations seemed to provide a clue for predicting the source of PMB: for example, S. aureus together with gram-negative facultative rods usually arose from a skin or soft tissue source, whereas S. faecalis together with a gram-negative bacillus could often be traced to an intraabdominal infection. No unique clinical features appeared to predict the occurrence of bacteremia due to multiple rather than to a single organism. The mortality in patients was 21%, lower than has previously been described in PMB but similar to that reported for bacteremia due to a single organism.