The purpose of this study was to compare the oral fluoride (F) retention following acidulated phosphate fluoride (APF) foam and gel applications in children. Fifty-nine children were divided into two groups, 6-9 years old and 10-13 years old. They each received a randomly assigned sequence of topical F foam and gel applications in two consecutive visits, using either sponge-lined or unlined trays. Approximately 0.6-0.8 g of APF foam, or 3-4 g of APF gel was used in each of the 4-min F applications. The amounts of F applied, recovered from the mouth, and retained were calculated for each treatment. The retention of an average of 1.26 mg F after an APF foam application was significantly less than 2.53 mg F for the APF gel. The sponge-lined trays also reduced significantly the F retention, compared with unlined trays. However, there was no significant difference in the amount of F retained between the two different age groups. The amount of F retained was reduced significantly from 3.65 mg F in the first appointment, to 1.24 mg F in the second appointment when unlined trays were used.