BACKGROUND Acute respiratory infections (ARIs) are the fourth major cause of morbidity and mortality among children under 5 worldwide. However, information on the risk factors that could predispose children under 5 to ARIs is scanty in Nigeria. OBJECTIVE To assess which environmental factors contribute to the acquisition of ARIs in children under 5 in Ibadan. METHODS A case-control study was undertaken. 220 children under 5 with ARI (cases) and 220 without ARI (controls) were selected consecutively from children visiting Oni Memorial Children's Hospital and University College Hospital, Ibadan. A pre-tested six-section questionnaire was administered to mothers to elicit information on factors that contribute to the acquisition of ARIs in children under 5. RESULTS Mean (SD) ages of cases and controls were 20·4 (14·7) and 20·3 (15·0) months, respectively. Mean (SD) household size of cases was 6·0 (1·5) compared with 4·0 (1·7) for the controls (P = 0·01). Children whose homes had more than two persons per room were found to have a 14-fold greater risk of ARI than children in houses with fewer than two persons per room. The use of a lantern at night (OR 4·1, 95% CI 2·4-6·9) and firewood for cooking (OR 9·3, 95% CI 3·6-24·1) were found to be independent risk factors for ARIs. CONCLUSIONS Cases were more exposed than controls to environmental risk factors for ARIs. Increased awareness of the importance of environmental factors with regard to prevention and control of ARI is therefore important.