We performed a hospital-based study to examine a hypothesis that indoor air pollution was associated with acute asthma in young children living in Kuala Lumpur City. A total of 158 children aged 1 month to 5 years hospitalized for the first time for asthma were recruited as cases. Controls were 201 children of the same age group who were hospitalized for causes other than a respiratory illness. Information was obtained from mothers using a standardized questionnaire. Univariate analysis identified two indoor pollution variables as significant factors. Sharing a bedroom with an adult smoker and exposure to mosquito coil smoke at least three nights in a week were both associated with increased risk for asthma. Logistic regression analysis confirmed that sharing a bedroom with an adult smoker (OR = 1.91, 95% CI 1.13, 3.21) and exposure to mosquito coil smoke (OR = 1.73, 95% CI 1.02, 2.93) were independent risk factors. Other factors independently associated with acute asthma were previous history of allergy, history of asthma in first-degree relatives, low birth weight, and the presence of a coughing sibling. There was no association between asthma and exposure to kerosene stove, wood stove, aerosol mosquito repellent, type of housing, or crowding. We conclude that indoor air pollution is an avoidable factor in the increasing morbidity due to asthma in children in a tropical environment.