In order to evaluate the possible cariostatic effect of a fluoride varnish in the primary dentition, Duraphat varnish was applied twice yearly for 2 years to the teeth of 195 3 1/2-year-old children of both sexes attending nine preparatory schools in the Warsaw area. A control group of 127 children attending nine other preparatory schools was also examined. For each child a dmfs1 index (including "caries without macroscopic defect") and a dmfs2 index ("caries without macroscopic defect" excluded) were determined as well as a dmft1 and a dmft2 index. After 2 years the difference between the test and the control groups regarding the mean dmfs1 and dmft2 index increments was 9.39% (P greater than 0.1) and 24.09% (P less than 0.05), respectively. No significant differences were found, however, concerning the dmft2 and dmfs2 indices, the percentage children with new gingival caries, the number of teeth extracted on account of caries and the number of remaining teeth. A test group of 46 children treated with fluoride varnish in which a complete radiological examination could be carried out before and after the experimental period and which was treated with fluoride varnish was compared with a similar group of 27 children, which was not treated with fluoride varnish. The differences in caries increments were small, less than 15%, and were not significant. Taking into consideration the great variation between the schools it was concluded that the cariostatic effect of Duraphat application twice yearly in the primary dentition of these children was slight.