To compare the clinical features of patients with the Lambert-Eaton myasthenic syndrome (LEMS) associated with carcinoma, with patients having LEMS but no cancer, reports on LEMS patients were analyzed systematically. Cancer was detected (CD group) in 62% of the 227 included cases. This CD group showed a male predominance (70%). No sex difference was found in patients in whom no cancer was detected (NCD group). Median age at onset of LEMS in the CD group was higher than in the NCD group (58 and 49.5 years, P<0.01). Median interval between onset of symptoms and diagnosis of LEMS was longest in NCD cases (P<0.001). CD patients had additional immunological disorders less frequently than NCD cases (6 and 27%, P<0.001). Symptoms distinguishing the CD group from the NCD group were weight loss (P<0.001) and need for prolonged artificial ventilation after anaesthesia (P<0.05). This analysis shows significant differences between CD and NCD cases of LEMS. The male predominance and higher age at onset in patients with a tumor probably reflects the characteristics of patients with small cell lung cancer. The high frequency of additional immunological disorders in patients without malignancy, together with the younger age at onset suggests a similar etiology as other non-paraneoplastic autoimmune diseases.