A clinical trial of one application of Nuva-Seal fissure sealant was conducted under School Dental Service conditions in a low-income area of London. The sealant was applied to one of a cariesfree pair of permanent teeth by schools' dentists, the other tooth being left as a control. One examiner, who did not apply the sealant, conducted examinations at baseline, after 6 months, and after 2 years. There were 118 children, initially aged 5-12, who remained in the trial after 2 years. The sealant was fully retained on 27.4 % of the teeth, partially retained on 40.6 %, and missing on 17.9 %. The remaining 14.1 % of teeth had been restored. There was no statistically significant difference between subjects who had less decay in sealed teeth and those who had less decay in control teeth. The sealant was 13.6 % effective (not significant, P less than 0.4) in preventing caries in sealed teeth. These results are less favorable than those in other trials reported. The potential role of fissure sealants in a School Dental Service is discussed.