Purpose: This before and after study evaluated the effects of a mandibular advancement device (MAD) on sleep bruxism (SB) activity and its associated signs and symptoms. Materials and Methods: Nineteen young adults (39.9 ± 12.9 years, 58% women) with a clinical history of SB without sleep or neurologic disorders and no spontaneous temporomandibular disorder pain were selected. SB activity was assessed after a habituation period of 2 weeks. The results of a 3-month treatment with a thermoplastic monoblock MAD were compared to baseline using electromyogram polysomnography and the BiteStrip, a portable EMG device. Sleep disorders were assessed and validated against the polysomnography sleep assessment questionnaire (SAQ). Additionally, common signs and symptoms of SB were evaluated with the research diagnostic criteria for temporomandibular disorders. Occlusal force was compared to baseline using a cross-arch force transducer. Results: There was a significant improvement in both SB activity and sleep scores (including SB episodes per hour) according to the BiteStrip and the SAQ, respectively. There was also a significant reduction in the signs and symptoms of SB, including grinding and/or clenching, temporomandibular joint (TMJ) sounds, muscle pain, and occlusal force. None of the SB subjects experienced MAD breakage, but in 24% of patients, the MAD treatment had to be interrupted due to TMJ/muscle pain and/or discomfort. Conclusion: The MAD treatment resulted in the reduction of SB activity, SB signs and symptoms, sleep disorders, and occlusal force. Int J Prosthodont 2014;37:119-126. doi: 10.11607/ijp.3675