Magnetic resonance (MR) imaging offers a noninvasive method of evaluating the mediastinum in infants with suspected compression of the trachea by the innominate artery. MR imaging was performed in 17 infants and children, aged 3-29 months, who had obstructive airway symptoms and who had undergone flexible fiberoptic tracheobronchoscopy. MR and endoscopic findings correlated in all cases but one. Of the remaining 16 patients, eight had endoscopic and MR evidence of tracheal compression at the level of the innominate artery, and eight had a normal upper thoracic trachea. In both groups, the innominate artery was situated anterior and to the left of the trachea with no difference in anatomic relationships. Measurements of the anteroposterior and transverse dimensions of the upper part of the mediastinum and the thymus failed to disclose significant differences between the two groups. Follow-up examinations of three patients showed decreasing tracheal compression without changes in tracheal-arterial relationships. This investigation disclosed no evidence of excessive mediastinal crowding. The findings support previous angiographic observations indicating that anterior tracheal compression is not due to an anomalous position of the innominate artery. Symptoms of tracheal compression may result from an intrinsic deficiency of the tracheal cartilage rather than an anatomic abnormality of the mediastinum or its vasculature.