The authors studied over a period of two years prospective connections between long-term artificial respiration and infections of the lower respiratory tract. A distinction was made between colonisation (RTC) and infection (RTI). Besides the incidence rate of RTC and RTI the severity of the infection was recorded. The connections between the underlying disease, the duration of artificial respiration, the age of the patient and the antibiotic treatment with the pathogenesis of RTC and RTI were studied. In more than half of the patients RTC could be demonstrated already within the first 24 hours; differences were related to underlying disease, start of ventilation and antibiotic treatment. The RTI incidence rate in all 104 ventilated patients was 67.3%. 19.2% of these already had RTI when respiration was initiated. Thus 48.1% of the patients developed RTI during artificial respiration. The diagnosis RTI was most frequent on the fourth day of respiration, and 80% of all RTI cases had appeared by the fifth day. The highest incidence was seen in patients with a thoracic trauma (85.7%). This group of patients also included the majority of life threatening cases of RTI. In one-quarter of the patients who developed RTI during artificial respiration there was no significant deterioration of pulmonary gas exchange; in 56% however, respiration was impaired to a life-threatening extent. The age of the patients did not have any bearing on the incidence rate and severity of RTI; however, all patients under artificial respiration who were over 70 years of age, died if the severity of the RTI had to be classified as life-threatening.(ABSTRACT TRUNCATED AT 250 WORDS)