The aim of this study was to statistically compare the retention of a chemically polymerized sealant (Delton, Johnson & Johnson Co., East Windsor, N.J.) with a widely tested UV-polymerized system (Nuva-Seal, L.D. Caulk Co., Milford, Del.) Two hundred children aged 5-16 years who resided in Fairfax County, Virginia (a fluoridated community) and who had one or more pairs of homologous permanent posterior teeth free of decay or fillings were admitted to the study. Posterior teeth on one side of the mouth of each subject were randomly assigned to receive Nuva-Seal; teeth on the other side were treated with Delton. Treated teeth were examined for sealant loss approximately every 6 months for a period of 2 years. Scores for complete retention of Nuva-Seal were 86% after 6 months and 78% at the end of 2 years. The corresponding values for Delton-treated teeth were 96% and 92%. For both sealants, scores of total retention were highest in mandibular first premolars and lowest in mandibular second molars. The greatest rate of sealant loss occurred within the first 6 months after placement. A comparison of overall retention scores (net gain) for Delton and Nuva-Seal revealed that Delton scores were significantly higher than Nuva-Seal at each semi-annual examination. The McNemar test was used to compare the retention of the two sealants for each tooth type. The test showed that the retention scores for Delton were significantly higher than those for Nuva-Seal only on maxillary and mandibular first molars.