This study was designed to explore the physical characteristics and child care practices of child care centres as potential risk factors for respiratory infections and diarrhoea in children 3-35 mo of age. A dynamic cohort averaging 667 children from 40 child care centres in Campinas (Brazil) was followed up for 8 mo. Direct structured observations were made to collect information on the centres' physical structure, child activities and child care practices. Despite their good infrastructures, hand washing in the centres was infrequent, and nearly 30% of diaper changes were unhygienic. Risk factors for diarrhoea included the presence of flies during meals, infrequent child hand washing before meals, and infrequent child hand washing after defecation, with relative risks of 1.33, 1.73 and 1.63, respectively. There was 33% less diarrhoea in classes where soap was frequently used during diapering. Contrary to our hypotheses, ventilation, group size and child density were not associated with respiratory infections, as was the case for most other factors studied. Only mixing of groups and a larger classroom area were found to be negatively associated with lower and upper respiratory infections, respectively. The results indicate that improving hygiene practices is the only intervention in child care centre characteristics with good potential to reduce diarrhoea among attendees. For respiratory infections, no modifiable characteristic with intervention potential was identified.