[Purpose] The purpose of this study was to examine the effects of smoking on teenagers’ internal oblique (IO) and transverses abdominis (TrA) expiratory muscles and their pulmonary function. [Subjects] A total of 30 healthy teenagers (15 smokers; 15 nonsmokers) voluntarily participated in the study. [Methods] The subjects were instructed to maintain an upright standing posture with their scapulars against the wall. Measurements were then taken to determine the thickness of their right IO and their right TrA while they were at rest and in a state of forced expiration using a 7.5 MHz linear probe of an ultrasonic imaging system. The measured thickness was converted into the percentage of change in muscle thickness (PCMT) and the relative contribution ratio (RCR) using a calculation formula, and then the data were analyzed. [Results] No significant differences were found between the two groups in the thickness, PCMT, and RCR of both the IO and TrA muscles, while there were significant differences in the forced expiratory volume at one second (FEV1) and the peak expiratory flow (PEF). [Conclusion] This study examined teenage smokers whose duration of smoking was relatively short. The two groups did not show significant differences in the thickness of both the IO and TrA muscles. However, based on the forced expiratory volume at one second (FEV1) and PEF measurements, the smokers showed greater decreases in pulmonary function than the nonsmokers.