We performed a case control study to examine protective and risk factors for acute respiratory infections (ARI) in hospitalized children in Kuala Lumpur. Consecutive children between the ages of one month and five years hospitalized for pneumonia (n = 143), acute bronchiolitis (n = 92), acute laryngotracheobronchitis (n = 32) and empyema (n = 4) were included as cases and were compared with 322 children hospitalized during the same 24 hour period for non-respiratory causes. Potential risk and protective factors were initially analysed by univariate analysis. Logistic regression analysis confirmed that several home environmental factors were significantly associated with ARI. The presence of a coughing sibling (OR = 3.76, 95%CI 2.09, 6.77), a household with more than five members (OR = 1.52, 95%CI 1.03, 2.19) and sleeping with three other persons (OR = 1.45, 95%CI 1.00, 2.08) were independent risk factors. Significant host factors were history of allergy (OR = 2.50, 95%CI 1.74, 3.61) and ethnicity (Malay race) (OR = 2.07 95%CI, 1.27, 3.37). Breast feeding for at least one month was confirmed as an independent protective factor (OR = 0.58, 95%CI 0.38, 0.86). However, the study was not able to demonstrate that domestic air pollution had an adverse effect. This study provides further evidence that home environmental factors, particularly those associated with crowding, may predispose to ARI and that breast feeding is an important protective factor.