Thomsen and Becker disease are myotonic disorders characterised by slow muscle relaxation associated with hyperexcitation of the muscle fibres occurring within the first few months after birth. The myotonia is unusual in that it is relieved by exercise (warm-up effect). Autosomal dominant myotonia congenita (Thomsen disease) is a non-dystrophic muscle disorder caused by mutation in the gene encoding skeletal muscle chloride channel-1 (CLCN1). It is clinically characterised by muscle stiffness and an inability of the muscle to relax after voluntary contraction. Autosomal recessive myotonia congenita (Becker disease) is caused by mutation in the gene encoding skeletal muscle chloride channel-1 (CLCN1). It is a non-dystrophic skeletal muscle disorder characterised by muscle stiffness and an inability of the muscle to relax after voluntary contraction. Most patients have symptom onset in the legs, which later progresses to the arms, neck, and facial muscles. Many patients show marked hypertrophy of the lower limb muscles. Transient muscle weakness is a characteristic feature.