BACKGROUND: Although many investigators have assessed the technique of endotracheal tube suctioning, the tissue of how frequently it should be performed remains controversial. No objective data is available to determine the answer. OBJECTIVE: To determine the criteria nurses use to make decisions regarding endotracheal suctioning. METHOD: Twenty-four registered nurses of varying levels of experience were asked a series of open-ended questions related to their theoretical knowledge, their ability to apply this knowledge, their suctioning practice, and how they made decisions regarding suctioning. RESULTS: All participants believed their patients required suctioning every 1 to 3 hours, with 17 performing it every 2 to 3 hours. Twenty criteria of which nine related to frequency were used in determining suctioning requirements. Eleven related to recognizing the need for immediate suctioning and were concerned with changes in the patient's condition. Common significant changes included desaturation, hemodynamic changes, loss of color, and blood gas changes. No single criterion was identified by all nurses. No differences were apparent between experienced and less experienced nurses. CONCLUSIONS: The large number of criteria identified suggests that the decision to suction is a complex one. It is a concern that many nurses rely on a deterioration in the patient's condition to indicate when suctioning is required. A policy of suctioning as necessary is provision of clear guidelines and education of all staff.