To evaluate the effects of lung disease on deposition of inhaled ultrafine particles (less than 0.1 micron diameter), we measured total respiratory tract deposition of nonhygroscopic particles of 0.02 to 0.24 micron in five subjects with obstructive lung disease and three subjects with restrictive lung disease and compared it with that in ten normal subjects. Deposition was measured as concentration difference of five size fractions in inhaled and exhaled air using an electrical aerosol analyzer. The data showed that deposition of these ultrafine particles was increased in subjects with obstructive lung disease when compared with normal subjects, while it was unchanged in subjects with restrictive lung disease. The increase in deposition in the subjects with obstructive lung disease was significant for particle sizes 0.04 to 0.24 micron. Possible mechanisms for increased deposition in airway obstruction include increased transit time of particles, abnormal expiratory collapse of airways due to flow limitation, and flow perturbations resulting from decreased airway caliber.