Critically ill children often require endotracheal intubation prior to transport to a medical center. Correct endotracheal tube placement and maintenance during transport are essential. The utility of a portable colorimetric end-tidal CO2 detector during transport of critically ill children was evaluated. Fifty-eight children with spontaneous circulation (aged 1 day to 12 years, weight 0.9 to 26 kg) who underwent 59 intubations during transport by ground (n = 31) or air (n = 27) were studied. Tube position was confirmed by physical examination, arterial blood gas values, or arterial oxygen saturation, and sometimes by chest radiography. The detector was attached and readings were obtained after intubation; readings were repeated if endotracheal tube position was rechecked during transport. Fifty-seven of 58 tracheal positions and the 1 esophageal tube position were correctly identified. One false-negative result occurred in a severely hypocarbic 900-g premature newborn. On each occasion that the detector was used en route, the endotracheal tube position was correctly identified. It is concluded that the end-tidal CO2 detector is a useful tool for confirming endotracheal tube position during transport of critically ill children weighing more than 2 kg who are not in cardiopulmonary arrest.