There are unconfirmed, reported differences in fluoride concentration in the tooth surfaces of male and female children whose average age was 13.5 yr (range 11.5-15.7 yr) and whose teeth might therefore have been exposed for 2-3 yr after eruption to the oral environment. Thirty-two unerupted and 24 erupted mandibular first premolars were now examined. These had been extracted for orthodontic reasons from children aged from 9 to 10 yr. Samples were removed by acid etching from small areas of the enamel surface, and the fluoride and phosphorus concentrations determined with an electrode and by colorimetry, respectively. Three sites on the buccal surface and one site in the centre of the lingual surface were investigated. Fluoride concentrations were higher in erupted than in unerupted enamel. The fluoride concentration of erupted enamel from the female teeth was significantly higher than that of the males (in contrast to the previous findings), although no analogous differences emerged in the fluoride concentrations of the unerupted enamel.