The effects of spring-loaded posterior bite-blocks on masticatory muscles were investigated to evaluate the correlation between masticatory muscles and craniofacial form in long-faced children. The appliance was used in 10 subjects (6 girls and 4 boys) with a chronological mean age of 10.40 +/- 1.12 years for the treatment of skeletal anterior open bite. Electromyographic activity of the anterior and posterior temporal and masseter muscles was recorded before and after treatment during postural position, maximal biting, chewing, swallowing, postural position with the appliance in the mouth, and maximal biting with the appliance in the mouth. After treatment, increases in SNB and overbite (P < .05, P < .001, respectively) and decreases in ANB, SNGoAr, and overjet parameters were found to be statistically significant (P < .05, P < .01). When the measurements related to muscle activity were examined, increases in anterior temporal postural (P < .05), anterior temporal chewing (P < .01), masseter chewing (P < .05), posterior temporal chewing (P < .05), and masseter swallowing (P < .01) were found to be statistically significant. A positive correlation was found between ANB and anterior temporal postural and a negative correlation between SNGoAr and masseter swallowing. The increase in muscle activities was considered to occur as a result of the appliance used.