To determine the incidence, characteristics, and course of polymicrobial sepsis among infants in intensive care nurseries, we reviewed all such episodes in our neonatal unit from September 1971 through June 1986. We identified 15 episodes (3.9% of all cases of culture-proven sepsis during the survey period) in which blood or cerebrospinal fluid (CSF) culture yielded multiple organisms felt to represent true pathogens. Mortality associated with late-onset polymicrobial sepsis (7 of 10; 70%) was significantly higher (P less than .001) than in late-onset monomicrobial sepsis (86 of 370; 23%). Six patients were 37 weeks' gestation or greater at birth, and five were younger than 4 days of age when the polymicrobial culture was obtained. Group D streptococci were recovered in eight cases (53%). Gastrointestinal foci appeared to be common among infants with late-onset polymicrobial infection (5 of 10), while prolonged rupture of membranes was frequently associated with early-onset infection (4 of 5). Though recovery of multiple organisms from blood or CSF may not always be significant, one should not immediately assume contamination. A report of more than one organism growing from a normally sterile body fluid in an intensive care nursery infant should be considered significant, and therapy should be adjusted to provide appropriate antimicrobial agents for all reported organisms if the infant has not substantially improved in the interval since the culture was actually obtained.