Patients with Proteus mirabilis bacteremia at a large community teaching hospital during a 13-year period (1980-1992) were retrospectively reviewed. There were 176 patients whereof 44 (25%) had polymicrobial bacteremia. The median age of patients was 75 years and 64.2% were 70 years or older. Of the patients 56.8% came from a nursing home, 64.2% had an indwelling Foley catheter, and 20.5% nosocomial bacteremia. The most common source of bacteremia was the urinary tract (52.8%) whereas the lower respiratory tract was an uncommon source. Hypotension was present in 30.1% of patients. The overall mortality rate was 29.0% with an attributable mortality rate of 25.6%. The mortality rate of polymicrobial bacteremia (38.6%) was higher than that of monomicrobial bacteremia (25.8%). For patients with monomicrobial bacteremia, significant increase in mortality was found in patients who had nosocomial infection (p < 0.02), whose source of bacteremia was other than the urinary tract (p < 0.01), who had ultimately fatal underlying conditions (p < 0.01), who had hypotension (p < 0.001), who had increase in serum creatinine (p < 0.05), or who had increase in serum bilirubin (p < 0.05).