Objective: To describe the pattern of defecation in critically ill ventilated patients and the influence of selective decontamination (SDD) and other medication. Design: Descriptive cohort study. Setting: Mixed surgical-medical ICU in a university Hospital. Patients: Ventilated patients with a length of stay ≥ 7 days taking part in a study on SDD. Measurements: Daily registration of defecation, SOFA (sepsis-related organ failure assessment score) score, administration of dopamine, noradrenaline, morphine and other medications. Results: The first defecation occurred after a mean of 6.2 days. Patients with defecation within 6 days had lower mean SOFA scores, received more cisapride and lactulose and less dopamine, noradrenaline and morphine, and had a shorter duration of mechanical ventilation and ICU stay. On 57% of the days, no stools were produced; on 31% diarrhea, and on 12%, normal stools. Patients receiving SDD had more days with normal stools and less with diarrhea. Diarrhea was preceded by the administration of lactulose in the majority of patients. Conclusion: Time to first defecation correlated with severity of illness, vasoactive medication, administration of morphine, cisapride and lactulose, duration of mechanical ventilation and length of stay. Diarrhea seemed at least partially iatrogenic.