A form of adult onset 'bulbospinal muscular atrophy' of X-linked recessive inheritance is described in 10 patients from eight families. Muscle weakness in the limbs was mainly proximal and developed in the third to fifth decades of life, often preceded by muscle cramps on exertion and tremor of the hands. Weakness and fasciculation of the facial muscles and tongue were prominent. All the patients had gynaecomastia and some were infertile. Two had diabetes mellitus. Motor nerve conduction studies were normal but most patients had small or unrecordable sensory action potentials in the absence of clinical sensory loss. Plasma creatine kinase levels were considerably elevated and muscle biopsies showed neurogenic atrophy together with secondary myopathic changes. The importance of recognising this distinctive disorder in single cases (six of the present series) is emphasised.