Background: Pneumonia is the dominant killer of children under the age of five (toddlers) in the world, more than other diseases. The incidence of pneumonia in the Special Region of Yogyakarta shows that the city of Yogyakarta still occupies the highest incidence of pneumonia. Therefore it is important to know the cause of the risk factors for the occurrence of pneumonia. One of them is by looking at the risk factors of pneumonia. The purpose of this study was to find out and prove that there are dominant risk factors for the incidence of pneumonia in infants in the Yogyakarta City area. Subjects and Methods: This study was a non-experimental descriptive with cross sectional approach. The study was conducted in July 2017 - July 2018 in PKU Muhammadiyah Hospital, Yogyakarta City with a purposive sampling of 152 respondents. Bivariate data analysis using the Chi-Square test. Results: The Chi-Square test results showed that there were several risk factors that showed a significant relationship to the incidence of pneumonia in children in the city of Yogyakarta, including Toddler Nutrition (PR=4.237 CI95%=2.243-5.324 p=0.043), Basic Immunization (PR=6.246 CI95%=2.963-13.167 p=0.045), Air Pollution (PR=3.188 CI95%=2.092-4.857 p=0.000), and Cigarette Smoke Exposure (PR=2.687 CI95%=2.282-3.221 p=0.000) Conclusion: There are dominant risk factors for the incidence of pneumonia in children under five in the city of Yogyakarta, namely toddler nutrition, basic immunization, air pollution and exposure to cigarette smoke.