The objective of this study was to assess the effectiveness ot a hygiene program in reducing the incidence of respiratory and diarrheal diseases in toddlers attending day care centers. A randomized field trial was conducted in 52 day care centers in Québec, Canada, between September 1, 1996 and November 30, 1997. Absences for any reasons and the daily occurrence of colds and/or diarrhea in toddlers were recorded on calendars by the educators. The number of fecal coliforms on children's hands and on educators' hands was measured during three unannounced visits. Overall, 1,729 children were followed in 47 day care centers for a total of 153,643 child-days. The incidence rate of diarrhea was considerably reduced by the effect of monitoring (IRR = 0.73, 95% CI = 0.54,0.97), and the intervention reduced the incidence rate of upper respiratory tract infections (IRR = 0.80, 95% CI = 0.68,0.93). Monitoring alone also had an important effect in reducing the level of bacterial contamination on children's and educators' hands. The results indicate that both an intervention program and monitoring alone play a role in reducing infections in children attending day care centers. (Epidemiology 1999;10:219–227)