Fluctuating stiffness and paroxysmal spasms of the trunk and legs are the primary features of stiff-man syndrome and it's variants, progressive encephalomyelopathy with rigidity and myoclonus (PERM) and stiff-leg syndrome. The spasms characterized by hyperextension of the back and legs are both spontaneous as well as stimulus-sensitive. They can be excruciatingly painful and are frequently accompanied by symptoms of autonomic dysregulation. Hyperreflexia may be the only pathological finding on the neurological examination. Most patients show psychiatric disturbances suggestive of psychogenic movement disorder and this may cause delays in adequate pharmacotherapy. The disease progresses over the span of months to years rendering many patients wheelchair-bound or bedridden. GABA-mimetics are most effective in treating symptoms, but tolerance and life-threatening withdrawal symptoms are common drawbacks. For therapy-refractory patients, intrathecal baclofen represents a good alternative. The diagnosis is based on clinical, biochemical and electrophysiological findings. Spasmodic reflex myoclonus is observed in nearly all SMS patients. It consists of well-reproduced reflex EMG-activity commencing 50-80 ms after medial or tibial nerve stimulation and lasting several seconds thereafter. The activity is first myoclonic then spasmodic in nature, and commonly begins in the muscles most severely affected before spreading bidirectionally along the neuraxis. Spasmodic reflex myoclonus and the high incidence of antibodies against GAD are suggestive of an autoimmune disorder affecting GABAergic neurons in the spinal cord, but the precise locus of dysfunction remains to be elucidated.