Bronchiolitis is the most common viral infection of the lower respiratory tract in infant in their first year of life with maximum incidence between 3-9 month of age and bronchiolitis is a major cause of morbidity and mortality in early childhood wideworld. The aim of the work is to assess the role of breast feeding and passive smoking in the development of bronchiolitis in infants. Methods: This case control study was conducted at Specialized Children's Hospital Cairo University during the period from November 2011 to April 2012. The study population included 100 infant in the first year of life presented with bronchiolitis who were recruited from the inpatient departments and emergency units along with 100 infant age and sex matched controls. Diagnosis of this patient was based on careful history taking and clinical examination. The present study was done by using a written questionnaire that take taken by infant's mother to asses the risk and protective factors and scoring system used to asses degree of bronchiolitis infants. Results: In this study, the significant risk factors (p value<0.05) were maternal age<20 years old, (51% in cases and 24% in control group), non educated mothers (34% in cases and 17% in control group), crowding index > 2 in room (52% in cases and 19% in control), artificial feeding (50% in cases and 11% in control),cesarean delivery (51% in cases and 34% in control) and exposure to passive smoking (69% in cases and 48% in control). Protective factors were exclusive breast feeding (31% in cases and 55% in control group), maternal education (66% in cases and 83% in control), maternal age ranged 20- 40 years old (33% in cases and 45% in control) and >40 years old (16% in cases and 31% in control) and vaginal delivery (49% in cases and 66% in control). There was a significant positive correlation between severity scoring and exposure to passive smoking (mild cases were 17.3%, moderate 42.2% and severe were 40.5%). There was a significant negative correlation between severity scoring and breast feeding (mild cases were 68%, moderate 20.5% and severe were 11.5%).Conclsion: Breastfeeding is a protective factor and passive smoking is a risk factor for bronchiolitis.