Response to treatment with antihistamines in a family with myotonia congenita. In a family in which myotonia congenita was found in five generations, both great-grandparents of the index case were affected. In subsequent generations mild and severely affected cases were clearly segregated down parallel lines of this family. The grandmother of the index case had noted improvement with an antihistamine. When the index case was prescribed trimeprazine, she showed a striking reduction in severity of symptoms. Antihistamines seem to deserve further evaluation as a safe and effective treatment for myotonia congenita. 