This article reports a case of masticatory muscle pain and progressive limited mouth opening secondary to amyotrophic lateral sclerosis (ALS), popularly known as Lou Gehrig's disease. The symptoms were first mistaken as those of temporomandibular disorders, before fatty degeneration of all masticatory muscles were discovered on magnetic resonance imaging (MRI). ALS should be considered in the differential diagnosis process when the patient presents with longstanding progressive mouth opening limitation associated with pain. MRI could facilitate the diagnostic process. Keywords: amyotrophic lateral sclerosis, motor neuron disease, mouth opening limitation, orofacial pain