Objective: To quantify the risks of malignancy and mortality in people with coeliac disease compared with the general population. Design: Population based cohort study. Setting: General practice research database. Participants: 4732 people with coeliac disease and 23 620 matched controls. Main outcome measures: Hazard ratios for malignancy and mortality. Results: Of the 4732 people with coeliac disease, 134 (2.8%) had at least one malignancy and 237 (5.0%) died. The overall hazard ratios were: for any malignancy 1.29 (95%confidence interval 1.06 to 1.55), for mortality 1.31 (1.13 to 1.51), for gastrointestinal cancer 1.85 (1.22 to 2.81), for breast cancer 0.35 (0.17 to 0.72), for lung cancer 0.34 (0.13 to 0.95), and for lymphoproliferative disease 4.80 (2.71 to 8.50). The increased risk was primarily in the first year after diagnosis, with only the risk for lymphoproliferative disease remaining significantly raised thereafter. After excluding events in the year after diagnosis, the hazard ratio for malignancy was 1.10 (0.87 to 1.39) and for mortality was 1.17 (0.98 to 1.38), giving absolute excess rates of 6 and 17 per 10 000 person years, respectively. Conclusions: People with coeliac disease have modest increases in overall risks of malignancy and mortality. Most of this excess risk occurs in the first year after diagnosis. People with coeliac disease also have a noticeably reduced risk of breast cancer. The mechanism of this merits further attention as it may provide insights into the cause of this common malignancy. Objective: To quantify the risks of malignancy and mortality in people with coeliac disease compared with the general population. Design: Population based cohort study. Setting: General practice research database. Participants: 4732 people with coeliac disease and 23 620 matched controls. Results: Of the 4732 people with coeliac disease, 134 (2.8%) had at least one malignancy and 237 (5.0%) died. The overall hazard ratios were: for any malignancy 1.29 (95% confidence interval 1.06 to 1.55), for mortality 1.31 (1.13 to 1.51), for gastrointestinal cancer 1.85 (1.22 to 2.81), for breast cancer 0.35 (0.17 to 0.72), for lung cancer 0.34 (0.13 to 0.95), and for lymphoproliferative disease 4.80 (2.71 to 8.50). The increased risk was to be in the first year after diagnosis, with only the risk for lymphoproliferative disease significant significantly raised thereafter. After excluding events in the year after diagnosis, the haz ard ratio for malignancy was 1.10 (0.87 to 1.39) and for mortality was 1.17 (0.98 to 1.38), giving an absolute excess rates of 6 and 17 per 10 000 person years, respectively. Conclusions: People with coeliac disease have modest increases in overall risks of malignancy and mortality. Most of this excess risk occurs in the first year after diagnosis. People with coeliac disease also have a noticeably reduced risk of breast cancer. The mechanism of this merits further attention as it may provide insights into the cause of this common malignancy