Background: Pneumonia is one of the leading infectious diseases causing death in children under five, following diarrhea. Pneumonia has resulted in the deaths of 740,180 children under five in Indonesia. Various factors can exacerbate the severity of pneumonia in children, such as basic immunization status, exclusive breastfeeding, and exposure to cigarette smoke. Objective: This study aims to analyze the relationship between basic immunization status, history of exclusive breastfeeding, and exposure to cigarette smoke with the severity of pneumonia in children under five. Methods: This research was conducted using an analytical observational method with a cross-sectional approach. The dependent variable is the severity of pneumonia. The independent variables are basic immunization status, history of exclusive breastfeeding, and exposure to cigarette smoke. The study sample comprised 35 children who met the inclusion and exclusion criteria, selected using consecutive sampling techniques. The research was conducted in the Seruni ward of Soewandhie Hospital, Surabaya, utilizing medical records and questionnaires. The data were analyzed using Fisher's Exact Test and Chi-square with a 95% CI (α = 0.05). Results: The majority of children experienced "pneumonia," with 23 children (65.7%). Statistical analysis showed no significant relationship between basic immunization status (p=0.632), history of exclusive breastfeeding (p=0.530), and exposure to cigarette smoke (p=0.618) with the severity of pneumonia in children under five. Conclusion: There is no relationship between basic immunization status, history of exclusive breastfeeding, and exposure to cigarette smoke with the severity of pneumonia in children under five at Soewandhie Hospital.