Diarrhoeal disease is a leading cause of morbidity in young children in rural Zaire. Few diarrhoea prevention programmes have been implemented in Bandundu Province, where available data suggest an annual prevalence rate of 10%. The urgent need to reduce diarrhoeal morbidity in Zaire, together with the potential effectiveness and feasibility of hygiene education as a diarrhoea prevention strategy, led to the development of the present research project. A randomized, controlled trial of an education intervention to reduce diarrhoea through improved personal and domestic hygiene behaviours was conducted in 18 geographically separate village clusters (sites) in rural Zaire. For 12 weeks baseline information on the diarrhoeal morbidity of 2082 children aged 3-35 months was collected at weekly home visits, and structured observations of hygiene practices related to diarrhoea were made on a subset of 300 families. Intervention messages addressed disposal of animal faeces from the yard, handwashing after defecation and before meal preparation and eating, and disposal of children's faeces. Three months after the start of the intervention and exactly 1 year after the baseline studies, a second diarrhoeal morbidity study and a second observational study were conducted in order to evaluate the intervention. Children in intervention communities experienced an 11% reduction in the risk of reporting diarrhoea during the peak diarrhoeal season, compared to controls (P < 0.025). The largest differences were seen among children aged 24-35 months, with those from intervention communities reporting significantly fewer episodes, shorter mean durations and hence fewer days of diarrhoea. There was some evidence that greater reductions in diarrhoea occurred in sites where the quality of the intervention, a scored measure of volunteer efficacy and community participation, was highest. The results of this study suggest that hygiene education may be an effective approach to reduce the incidence and duration of diarrhoeal episodes in rural Zaire. Children aged 2 years appear to benefit the most. A Hawthorne effect of the education may contribute to diarrhoeal reductions.