Cross-sectional household surveys are extensively used for data collection, priority setting and programme evaluation in developing countries. They are now being promoted to assess a number of health care interventions such as Control of Diarrhoeal Diseases (CDD), Expanded Programme on Immunization (EPI), AIDS control and child survival programmes. Few field studies of the validity and precision of data generated from these surveys have been carried out, in part because such work is rather demanding of resources. The purpose of this study was to draw conclusions on validity and reliability of data from household surveys through a comparison of results from large-scale surveys on diarrhoea conducted by CDD and Demographic and Health Surveys (DHS) programmes in developing countries. Diarrhoea prevalence and treatment were compared for nine surveys for which little time had passed in between the CDD and DHS survey. The variation in results between the surveys was in many instances too large to be explained only by a true variation in the variable studied. A literature review suggested that validity problems could be due to response and recall errors. The authors caution the use of frequent household surveys for programme impact evaluation. Their cost-effectiveness should be carefully assessed, especially when services' evaluations already have provided evidence that a programme has had a positive effect on the behaviour of health workers and target groups in the community. It is recommended that more research be carried out on how selection and training of surveyors can be improved to make the quality of household surveys in developing countries better.