A prospective epidemiological survey was carried out over a period of seven weeks in a medical intensive care unit. Bacteria from patients, staff and air were monitored and the transmission of isolated microorganisms was followed. Handwashing samples revealed pathogenic bacteria in 30.8% of physicians (average number of colony forming units: 71,300 per hand) and 16.6% of nurses (39,800 cfu per hand). Air cultures yielded pathogens in 15% of sampling periods and nine of 53 patients were found to be colonized with Gram-negative bacteria, Staphylococcus aureus or Candida spp. The spectrum of bacteria recovered from patients and air was generally different, whereas strains recovered from patients and their attendants' hands were indistinguishable on multiple occasions. The results of this study confirm that direct contact is the principal pathway of microbial transmission, whereas little evidence for a significant role of airborne transmission is shown. The call for more extensive air-filtering and ventilation systems in medical intensive care units is not supported by the results shown in this communication.