This case report describes a young man referred for electrodiagnostic evaluation for hand stiffness and intermittent numbness. His needle electromyography revealed diffusely increased insertional and spontaneous motor activity in the form of myotonic discharges. Given the finding of symptomatic myotonia also in his mother, Thomsen myotonia was suspected. Investigations not only confirmed Thomsen myotonia, but also led to the identification of a previously reported heterozygous Becker mutation in both the proband and his mother.