Rigidity in the setting of continuous motor unit activity at rest can be caused by a variety of central and peripheral conditions. A central origin is suggested by the presence of painful reflex spasms. Focal spinal lesions and infective causes are relatively easily excluded through imaging, microbiological and serological studies. There then remain a group of patients who may have the classical 'stiff-man syndrome' or a related syndrome. When strict diagnostic criteria are used, patients with the stiff man syndrome uniformly have axial rigidity, and about 90% are found to have antibodies against glutamic acid decarboxylase. Treatment response and prognosis are excellent. Stiff persons with 'plus' signs, particularly those with rigidity of a distal limb, are unlikely to have the classical stiff man syndrome. They have a poorer treatment response and prognosis. Some have a paraneoplastic aetiology, while a non-malignant autoimmune basis seems likely in others. Those in whom post-mortem pathology findings are available usually are seen to have had an encephalomyelitis with prominent involvement of the grey matter. Clinically, stiff persons with 'plus' signs may be divided into three groups according to the aggressiveness of the pathology and its relative distribution. Encephalomyelitis with rigidity follows a relentless subacute course, leading to death within 3 years. Chronic cases may present with predominantly brainstem involvement, including generalised myoclonus (the 'jerking stiff person syndrome') or spinal cord involvement, dominated by stiffness and spasm in one or more limbs (the 'stiff limb syndrome').