Myotonia is a symptom, which occurs in a series of hereditary diseases, and it is also seen in less frequently occurring syndromes. A summary is given of conditions with myotonia. Five cases are reported from a family with a dominant hereditary disease presenting myotonia, muscular hypertrophy and increased muscle irritability as the only symptoms. In the most affected patient, some unusual rolling muscle contractions are seen. Apart from a moderate increase of creatin kinase, supplementary examinations are normal. The clinical picture resembles myotonia congenita Thomsen, but differs from this in significant respects. Other diagnostic possibilities are also considered. It is concluded that the clinical picture is different from all previously described conditions.