OBJECTIVE--To provide accurate estimates of the risk of lymphoma in patients with dermatitis herpetiformis. DESIGN--Comparison of observed and expected incidence of cancer in patients with dermatitis herpetiformis. SUBJECTS--976 patients aged 4 to 97 years with no clinical signs of coeliac disease who were admitted to hospital between 1963 and 1983. SETTING--Data from Swedish Cancer Registry. MAIN OUTCOME MEASURES--Incidence and type of cancer. RESULTS--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). CONCLUSIONS--The risk of lymphoma is greater in male patients with dermatitis herpetiformis, and this calls for increased surveillance in these patients.