Polymicrobial blood or urine cultures in bacteraemic urinary tract infection (UTI) are relatively common. There is, however, very little information available on the clinical and bacteriological features that distinguishes between monomicrobial and polymicrobial urosepsis. During 1980-84, 68 of 198 episodes (34%) of urosepsis with at least one identical organism in blood and urine, had multiple growth in either one or the other. Comparison between monomicrobial and polymicrobial infectious episodes showed that the latter were more often hospital-acquired and more frequently associated with urinary catheters. Pseudomonas aeruginosa was more often associated with polymicrobial than with monomicrobial infections, whereas Escherichia coli was more common in monomicrobial infections. Mortality was higher in polymicrobial infections, and was further increased if multiple organisms grew from blood rather than from urine. Thus, there are clinical, microbiological and prognostic characteristics that distinguish polymicrobial from monomicrobial bacteraemic UTIs.