A study of knowledge and attitudes towards dental health of 666 youths in their early teens has been conducted in Quebec. Its purpose is to compare the effectiveness of two instructional approaches often used in dental health education: a sound and slide presentation or a verbal demonstration. This experiment also included a control group. The measures were two scores obtained from a knowledge and an attitude test, which were the two dependent variables. These measures were repeated on four occasions: pre-test, immediate post-test, post-test after two months, and post-test after six months. During the experimental months, no other form of dental education was given by community health workers. The statistically significant interaction of time with treatments allowed reaching the conclusion that the evolution of subjects was different according to the treatment they had received, considering both dependent variables simultaneously. The sound and slide presentation appeared to be better than the verbal presentation for improving dental knowledge in young adolescents. Both educational approaches enhanced adolescents' attitude towards better dental health for a short period, but results were no longer significant after two months, thus reinforcing the importance of repetition in dental health education. Measures for knowledge tests revealed significant differences between boys and girls.