Robert F. Kennedy Jr.s voice has been described as raspy and strained, and he has been public about being diagnosed with spasmodic dysphonia, a chronic neurological voice disorder and a focal laryngeal dystonia. Spasmodic dysphonia results in involuntary spasms of the muscles that open or close the vocal folds, causing a voice that presents with breaks and strained/strangled quality or breathy quality, depending on the type of spasmodic dysphonia (adductor or abductor). The exact cause of spasmodic dysphonia is not known, but it is known to be neurological in origin. There may also be a genetic component, as genes have been identified in other forms of dystonia.

Kennedy has explained that he had a very strong voice until he was 42 years old, but in 1996 he had an injury that caused spasmodic dysphonia. He has also said that he cannot listen to his own voice and apologizes to everybody who has to listen to him. Although there is no cure for spasmodic dysphonia, patients can undergo speech therapy or counseling.