The average charge per patient due to nosocomial infection for 215 nosocomial infections in 183 study patients was $693. These costs, however, were concentrated in very few patients; 5% of patients accounted for nearly one-third of total charges. The 10% of patients with highest nosocomial infection costs were patients on Medical or Surgical services; these services were utilized in 71% of patients with nosocomial infection and accounted for 86% of the attributable charges. Among the 22 most costly infections, 17 occurred in surgical wounds and lower respiratory tract. Although these sites accounted for 46% of the infections, they resulted in 77% of the total nosocomial infection charges. Patients with a primary diagnosis of injury had particularly costly infections. Combined analysis of these variables revealed two groups for whom nosocomial infections were especially costly: surgical patients who acquired wound infections after injuries, and medical patients with lower respiratory infections.