The clinical effectiveness of Delton fissure sealant was studied in 205 children, ages 6--10 years (mean age 71/2). 993 children in grades 1, 2, 3 and 4 were screened and subjects were selected if there was evidence of previous dental caries in the mouth, and a pair of contralateral maxillary or mandibular first permanent molar teeth were free of caries. Two pedodontists independently examined the children in a mobile van equipped with two operatories, and each applied sealants to approximately half of the subjects. After 11 months, in 186 subjects there were 10 sealants partially lost, five experimental teeth carious or filled and 53 control teeth carious or filled. After 24 months, in 175 subjects there were four sealants completely lost, 12 sealants partially lost, 11 experimental teeth carious or filled and 89 control teeth carious or filled. Consequently, sealant retention was 92% after 11 months and 85% after 24 months. Percent effectiveness in caries reduction was 90% after 11 months and 88% after 2 years.