This clinical study evaluated the retention rate and caries protection of a medium-filled (46% volume) flowable restorative material (CuRay-Match, OMNII Oral Pharmaceuticals, West Palm Beach, FL 33409, USA) compared to an unfilled sealant (Delton, Dentsply Caulk, Milford, DE 19963, USA). Using a half-mouth design, sealants were applied on randomly assigned caries-free first and/or second permanent molars of 32 children ranging in age from 6-11 years. A total of 118 teeth were etched, dried and sealed. Teeth were evaluated at one, six and 18-month intervals. After one month, 52 teeth sealed with unfilled sealant were intact compared with 46 sealed with a medium-filled resin, and after six months, 36 teeth sealed with an unfilled sealant were intact compared with 27 that were sealed with a medium-filled resin. After 18 months, 29 teeth were still fully sealed with an unfilled sealant, whereas 18 were sealed with a medium-filled resin. The difference between the two groups was not statistically significant. Regarding caries development, four teeth sealed with a medium-filled material and five teeth sealed with an unfilled sealant were decayed after 18 months. These results indicate that a medium-filled flowable restorative material did not perform better in retention rate and caries increment when compared to an unfilled conventional sealant. However, the effect of the additional techniques, such as the use of bonding agent and fissurotomy on retention rates should be evaluated in further studies.