SUMMARY A 68 year old woman is described with persisting muscular rigidity of the left lower leg together with transient myoclonic jerking in the left quadriceps muscle. Six weeks after onset a small cell carcinoma of the lung became manifest. With radiotherapy and chemotherapy complete remission was achieved. Segmental muscular spasm improved at the same time. Necropsy revealed loss and degeneration of alpha-motor neuron cells at one side of the anterior horn of the lumbar enlargement. This case may represent another manifestation of paraneoplastic subacute motor neuronopathy.Muscular rigidity due to spinal dysfunction and spinal myoclonus are thought to result from absent or disordered inhibition of alpha-motor neurons respectively."5 The presence of both these symptoms together has been reported in one case of encephalomyelitis.6 We describe another patient with symptoms of segmental rigidity and spinal myoclonus which were followed after 6 weeks by the appearance of a small cell carcinoma of the lung. Histology of the spinal cord offered an explanation for the clinical findings and indicated a paraneoplastic aetiology. examination an alert afebrile woman was seen who seemed exasperated by the pain in her leg. The left knee was held in flexion; stretching it exacerbated the pain in the calf. The left foot was fixed in plantar flexion, the big toe in extension. No active plantar or dorsal flexion of the foot or toes, was possible. On palpation both the soleus and tibialis anterior muscle groups were contracting. Sometimes regular myoclonic movements at a rate of about one contraction per second in the ankle or knee were seen, especially with the legs hanging. No fasciculations were seen. No specific sensory stimuli exacerbating the contractions could be found. Muscle power in the affected leg was difficult to assess; proximal muscle power was intact. Sensation was undisturbed except for a well demarcated dysaesthetic area at the level of D8 on the left. The patellar reflex on the left was increased. The remainder of the neurological examination, including the mental status, was normal. General physical examination revealed no other abnormalities.Laboratory studies gave normal results including WR, anti-nuclear antibody, circulating immune complexes and angiotensin-converting enzyme. Cerebrospinal fluid showed a normal cell count, elevated protein (0 55 and 0-66 g/l on different occasions), elevated IgG index (0 85, 0 75 and 2-07 on different occasions) with a small spike of gamma-globulin on electrophoresis which was not seen in serum. Cytology was negative. A chest radiograph was normal. Muscle biopsy from the left anterior tibial muscle showed foci of muscle fibres with a variable transverse diameter and locally degenerating nuclei. Scattered areas of polynuclear inflammatory infiltrate with some necrosis and histiocytes were present. The findings were thought to be compatible with intense continued muscular activity. The electromyogram showed normal action potentials with continuous motor activity in the le...