Unexplained neurological disorders sometimes develop in patients with cancer, even when the primary tumour or its metastases have not invaded the central nervous system. Visual dysfunction in patients with small-cell carcinoma of the lung is one of these paraneoplastic syndromes and is associated with serum auto-antibodies which react with specific subsets of retinal neurons. These sera have now been shown to react with cell lines derived from small-cell carcinoma. The antibodies recognise a small number of antigens shared by retinal and tumour cells, suggesting that an autoimmune response may be triggered by the tumour.