A 67-year-old man developed dermatomyositis. Investigation revealed an oat cell carcinoma of the lung. When he developed unilateral gynaecomastia this tumour was found to secrete gonadotrophins. The gynaecomastia disappeared after treatment with an anti-oestrogen--tamoxifen. This use of tamoxifen has not previously been described. Partial tumour regression was obtained with several cytotoxic drug regimes. His dermatomyositis responded to this treatment and remains in remission.