Adherence of mucoid and nonmucoid isolates of Pseudomonas aeruginosa to tracheal epithelium was quantitated by using hamster tracheas mounted in a perfusion chamber. The strains of P. aeruginosa used were clinical isolates from cystic fibrosis patients and a series of laboratory strains. Aseptically excised hamster tracheas were mounted in perfusion chambers and embedded in minimal essential medium containing 1.5% agarose. The tracheas were infected with various numbers of bacteria for various periods, rinsed, homogenized, and plated on Trypticase soy agar. A 4-mm segment from each trachea was prepared for quantitation, and the other segment was prepared for examination by scanning electron microscopy. Adherence increased with time and with increasing concentrations of inoculum. Standard conditions of inoculation were set at an inoculum of 10(7) CFU/ml and a 2-h incubation. Under these conditions, the mucoid organisms adhered to the ciliated epithelium 10- to 100-fold better than did the nonmucoid organisms. Adherence of the mucoid isolates did not appear to be pilus mediated and did not involve hydrophobic interactions. The mucoid P. aeruginosa isolates could be seen adhering to the epithelium in the form of microcolonies embedded in an extracellular matrix which attaches the organisms to the cilia and to each other. The adherence may be involved in the establishment of infection of the lungs of these patients and in the inability to clear the organisms from the lungs. The model will be useful in determining the mechanism of adherence of the bacteria to the ciliated epithelium of the respiratory tract.