OBJECTIVE: To examine the relationship between body mass index (BMI) and an inpatient diagnosis of coeliac disease (CD) in two independent Swedish national registers. MATERIAL AND METHODS: Study 1: Cohort study of women. The relationship between (pre-pregnancy) BMI and CD in pregnant women was evaluated (174 undiagnosed CD (at time of pregnancy), 550 diagnosed CD, 787,986 without a diagnosis of CD). The association between BMI and undiagnosed CD was estimated by Cox regression. Study 2: Case-control study of men. The relationship between BMI and CD in male conscripts was evaluated (70 undiagnosed CD, 1,047 diagnosed CD and 6,887 without a diagnosis of CD). The association between BMI and undiagnosed CD was estimated by logistic regression. Prevalence of underweight, normal weight and overweight was compared between diagnosed CD, undiagnosed CD and no diagnosis of CD. RESULTS: The prevalence of underweight (BMI <18.5) in women was: reference individual: 5.2%; undiagnosed CD: 16.7% and prior diagnosis of CD: 6.4%. In men, the corresponding figures were 6.5%; 14.3% and 9.8%, respectively. Underweight was associated with undiagnosed CD (future diagnosis of CD) in both women (hazard ration (HR) = 2.5; 95% CI = 1.6-3.7) and men (odds ratio (OR) = 2.4; 95% CI = 1.2-4.9). In women, overweight was negatively associated with undiagnosed CD (HR = 0.6; 95% CI = 0.4-0.9), but not in men (OR = 1.1; 95% CI = 0.6-2.2). 9.2% of women with undiagnosed CD and 14.3% of men with undiagnosed CD were overweight. CONCLUSIONS: Underweight individuals are at increased risk of having undiagnosed CD. However, overweight does not rule out CD.