122 multiple trauma patients staying in the ICU for 5 or more days and needing mechanical ventilation were investigated to determine the effect of selective decontamination of the digestive tract on prevention of infection. The (retrospectively studied) control group of 59 patients received no antibiotic prophylaxis. The infection rate during ICU-stay was 81%. Most infections were caused by potentially pathogenic microorganisms (PPM) from the oral cavity or the intestines (i.e. endogenous infections). The oropharynx and the intestines were rapidly colonised with ICU-associated gram-negative bacilli. After 2 weeks more than 80% of patients were found colonised. This secondary colonisation of the digestive tract is a very important stage in the pathogenesis of infections. Sixty-three patients were selectively decontaminated with nonabsorbable antibiotics, administered through the gastric tube even if peristalsis was absent. Emphasis was laid on the selective decontamination of the oral cavity, using topical application of an antibiotic paste. With this technique the oral cavity was free of PPM within 3 days in most patients. No secondary colonisation was found. Rectal colonisation decreased significantly after 5 days. Secondary colonisation occurred in 9 patients with PPM sensitive to the antibiotics used. The prophylactic regimen included systemic cefotaxim, directed against early endogenous infection. The suppression and after some time the absence of the endogenous source of PPM resulted in a significant reduction of colonisation and infection. The total infection rate decreased to 16%.