Myasthenia gravis is an autoimmune disease, manifested by progressive muscular weakness and fatigue. There are frequent ocular and bulbar symptoms, among them, dysphonia. This article reports a case of dysphonia that contributed to the diagnosis of myasthenia gravis through a speech evaluation. The patient sought speech therapy with the ENT diagnosis of presbyphonia. The perceptual voice assessment and acoustic analysis pointed out respiration, glottal voice source and resonance affections. Considering that some of the data obtained from anamnesis and vocal assessments were not directly related to presbyphonia, the speech therapist discussed the case with the physician and they both concluded it was necessary to refer the patient to a neurological evaluation. The neurologist then raised the diagnostic hypotheses of myasthenia gravis and requested further examinations. The patient underwent speech therapy and drug treatment. A vocal reassessment, which occurred two months after the initial treatment, showed improvement in voice quality, with great impact on quality of life. This article shows the importance of detailed clinical speech evaluation and participation of a speech therapist in an interdisciplinary team.