Mentors in early medical training have been reported to make a significant influence when resident physicians decide their specialties. Recently, the number of pulmonologists in Japan is considered insufficient for the rapidly increasing numbers of patients with respiratory diseases. To elucidate key factors of decision making by resident physicians on their future specialties, we surveyed current residents and junior pulmonary trainees with a questionnaire that focused on factors relating to their specialties decisions on what specialty they might undertake, including influences their mentors might have had. The results show that the chief factors of importance include satisfaction offered by the specialty, academic interest, and working with an excellent mentor. We recognized some differences in key factors for selecting specialties and disposition of an ideal mentor among first-year residents, second-year residents, and pulmonology junior trainees. To encourage more residents and junior trainees to select pulmonology as their specialties, we need to apply satisfaction and excitement to the work of pulmonologists. We also need to arouse the academic interest of trainees in this field, adjust training methods for each depending on medical experience and interests. In conclusion, established pulmonologists could play a pivotal role in the encouragement of future pulmonologists by brushing up their own skills and knowledge as “good role models” for young doctors.