To provide accurate estimates of the risk of lymphoma in patients with dermatitis herpetiformis. Comparison of observed and expected incidence of cancer in patients with dermatitis herpetiformis. 976 patients aged 4 to 97 years with no clinical signs of coeliac disease who were admitted to hospital between 1963 and 1983. Data from Swedish Cancer Registry. Incidence and type of cancer. 106 cancers were diagnosed in 94 patients. The relative risk was 1.4 (95% confidence interval 1.1 to 1.7) in male patients and 1.2 (0.8 to 1.7) in female patients. When the individual cancer sites were analysed a significant risk was found only for malignant, non-Hodgkin's lymphoma in male patients, with a relative risk of 5.4 (2.2 to 11.1). The risk of lymphoma is greater in male patients with dermatitis herpetiformis, and this calls for increased surveillance in these patients.