A clinicopathological study of the paraneoplastic neuromuscular syndromes associated with lung cancer. The highest incidence of remote neuromuscular disorders in cancer has previously been reported in lung carcinoma. The clinical incidence of neuromuscular disorder was estimated and correlated with muscle histology and the histological type of lung tumour in 100 patients with lung carcinoma who were studied prospectively. Thirty-five patients had small cell carcinoma and 65 patients non-small cell lung cancer. Clinically, 33 patients had a polymyopathy, of whom 18 had a cachectic myopathy and 15 had a proximal myopathy (two patients had Lambert-Eaton myasthenic syndrome, one presented with dermatomyositis and one had evidence of ectopic ACTH production). Cachexia was more common in non-small cell cancer; proximal myopathy was more common in small cell cancer. Ninety-nine patients had abnormal muscle histology; 74 had type II atrophy, 12 had type I and II atrophy, one had type I atrophy and 12 had necrosis. The majority of patients were affected sub-clinically and the clinical entities of cachectic and proximal myopathy did not correspond to previous pathological classifications. Atrophy was not related to the duration of tumour symptoms, ageing, clinical type of myopathy or histological type of lung tumour, and was statistically different from that seen in controls. Qualitatively, the presence of weight loss, muscle wasting and metastatic disease were not factors in the development of atrophy. Similarly, necrosis was not related to the type of lung tumour, the presence of metastases, ageing, weight loss, muscle wasting, duration of tumour symptoms or the clinical form of myopathy. This study demonstrates that lung carcinoma has a direct effect on the motor unit, including atrophy, a necrobiotic myopathy and Lambert-Eaton myasthenic syndrome. Clinical assessment does not accurately assess the 'remote' neuromuscular effects of cancer on the motor unit. 