METHODS Randomised controlled trial in primary care setting. METHODS 508 children aged 18-30 months from high caries areas of South Wales with caries-free first primary molars were recruited to the trial after informed consent. All children (n=508) received a standard package of dental health education. Children in the test group (n=241) had their first primary molars sealed with glass ionomer. All the children were re-examined once at varying intervals between 12 and 30 months. METHODS Decayed, missing and filled teeth (dmft) and presence of glass ionomer sealant. RESULTS Analysis of the caries data revealed no significant difference between test and control groups for any of the parameters examined. Examination of the confidence intervals showed no indication that there might be a difference even if the sample size had been increased. CONCLUSIONS There is no evidence that the intervention as used in this population had any effect on caries incidence and it cannot be recommended as a clinical procedure.