All of one year's 251 patients of a predominantly surgical intensive care unit (i.c.u.) were continuously followed up for infections according to a standard protocol. These protocols were evaluated for 174 patients who stayed at least 48 h at the unit. More than one third (36.7%) were already infected on admission (external origin), 35% contracted infections at the unit, primarily or additionally (internal origin) and 40% remained without an infection. Fifty eight percent of patients already infected on admission were surgical and required intensive care for complications. Among the patients who contracted their infection solely at the unit 61.5% suffered from trauma. Patients having contracted their infections at the i.c.u. stayed significantly longer than those without (additional) infections (13 and 6 days respectively). Mortality was highest (45%) in patients who were already infected on admission and who acquired additional infections during their stay at the i.c.u. Of patients with infections of exclusively external or internal origin 23.5 and 17.9% respectively died whereas among those who remained uninfected this proportion was only 7%. The 75 infections acquired before admission to the i.c.u. included infections of the respiratory tract in 14.4% of all patients, peritonitis with 10.3%, urinary tract in 8.0% and septicemia in 5.2%. Artificial ventilation was employed more often in infected patients (73.8-100%) than in non-infected ones (56.3%). They also carried more intravasal catheters (2.76-3.05 per patient) than the latter group (1.79). Of the 82 infections acquired in the i.c.u. the respiratory tract was affected in 19.5% of all patients and the urinary tract in 13.8%.(ABSTRACT TRUNCATED AT 250 WORDS)