Our study sought to determine whether tracheal dimensions were reduced in patients with Down's syndrome and whether such a narrowing, if present, was related to the presence of congenital heart disease or to body habitus (height or weight). The inner diameter of the tracheal air column was measured at 2 cm above the aortic arch in 14 adult patients with Down's syndrome, and the results compared with previously established norms. Two-way analysis of variance showed no significant difference in z-scores between sexes or between those with or without congenital heart disease. Z-scores were significantly different from zero for both coronal diameters (p = 0.0010) and sagittal diameters (p = 0.0003). The negative limits on the 95% confidence interval for coronal z-scores (-2.5 to -0.8) and sagittal z-scores (-2.6 to -1.0) indicate that our patients have tracheal diameters significantly smaller than normal. Linear regression analysis showed no significant correlation between tracheal diameters and patients' height or weight. We conclude that tracheal diameters in adult patients with Down's syndrome are reduced and that the narrowing cannot be ascribed to associated congenital heart disease or to body habitus.