Paraneoplastic neurological syndromes are of two types: some are more often seen without than with cancers and may therefore be called "occasionally para neoplastic" (e.g. chronic sensorimotor polyneuropathy and polymyositis), while others are fairly regularly associated with cancers, and particularly with small cell lung carcinoma. In this category falls subacute encephalomyelitis, an entity of broad anatomico-clinical spectrum including limbic encephalitis and subacute sensory neuronopathy; the patient's serum and cerebrospinal fluid may contain neuronal antinuclear antibodies. One type of subacute cerebellar degeneration is characterized by the presence of antibodies specifically directed against Purkinje cell cytoplasmic antigens, and it is associated with ovarian and mammary cancers. The other type shows no antibodies or different antibodies and sometimes neuronal antinuclear antibodies; the latter case may represent the cerebellar form of subacute encephalopathy. Because it may be either a true autoimmune disease or a true paraneoplastic syndrome, Lambert-Eaton myasthenic syndrome, caused by autoantibodies that block the voltage-dependent calcium channels, stands out as the most convincing argument in support of the autoimmune paraneoplastic syndrome theory. The theory, which refers to cross-antigenicity, cannot be extended to the other syndromes without reservation: there is no evidence that autoantibodies are neurotoxic, and specific autoantibodies in high levels are sometimes detected in patients with cancer but without any neurological symptom. Nevertheless, the finding of circulating antineuronal antibodies in patients with a suggestive clinical syndrome should prompt investigations for cancer perhaps at an early stage.