Passive smoking in the family is a major influence in the risk of lower respiratory infections in infants especially on bronchiolitis. Bronchiolitis is the most common lower respiratory tract infection that affects children in the first two years of life and the most common reason for hospitalization in this age group. Hence based on the above findings the present study was planned to assess the role of parental smoking in the development of severe bronchiolitis. The study was planned in the Upgraded department of Pediatrics, Patna Medical College & Hospital, Patna. Total 100 children were enrolled in the study group. This is a case –control study. Out of 100 children, 50 children with severe bronchiolitis were divided in Group A as cases and 50 were divided in the Group B as control. Chest radiograph was done for the evidence of air trapping in both lungs. Result: The mean age of patients was 7 (± 3.5) months. 32 (64%) patients were male and 18 (36%) were female. Male to female ratio was 1.7: 1. Most of the cases came from low socioeconomic background. More than half 56% of the cases were not exclusively breastfed. Thirty (60%) cases and eighteen (36%) controls were exposed to smoking. Result was highly significant (p = 0.005). Odds ratio was 2.5. Conclusion: Exposure to parental smoking causes a statistically significant (p = 0.005, odds ratio 2.5) increase in the risk of developing severe bronchiolitis in first year of life.