Two patients with an acute organic brain syndrome and accompanying neurological symptoms are described. Extensive work up showed that both patients suffered from small-cell lung cancer. Cerebral metastases were absent. Following chemotherapy and radiotherapy to the primary tumor one of the two patients showed a complete remission of psychiatric symptoms for one year. A paraneoplastic origin of this syndrome, in the literature known as limbic encephalitis, is postulated. The exact cause of this syndrome is yet unknown. Recent research reveals data indicating an immunological pathogenesis. The major clinical importance of this (neuro)-psychiatric syndrome is that its appearance may serve as a warning sign for an occult malignancy; furthermore, effective treatment of the primary malignancy can reverse the encephalitis. Thus antitumor therapy can result in a prolonged survival and considerably improved quality of life.