The volume of isoflow (VisoV) in 29 asymptomatic smokers with normal radiographic and pulmonary function studies was studied, with the purpose of investigating the presence of early alterations of the transitional airways in a group of 256 chronic smokers. The VisoV was measured using a body test plethysmograph, and two flow-volume curves with air and with helium were performed. 13 out of the 29 subjects (45%) showed an abnormal VisoV of between 17