Atypical and relatively silent forms of coeliac disease (CD) have been described in insulin-dependent diabetes mellitus (IDDM). Our aim was to evaluate the prevalence of CD-IDDM with serological markers and to investigate the presence of CD in the IDDM first-degree relatives. During 1993-94 we explored 116 IDDM patients reported as new cases and 381 first-degree relatives of IDDM patients. Determination of IgA and IgG antigliadin antibodies (AGA) and IgA antiendomysium antibodies (AEA) was made. Jejunal biopsy was performed in symptomatic patients or in those with positive serological markers, (i) Nineteen IDDM-CD were identified and 5 were suspected. Thus, the prevalence of CD in IDDM patients was between 16.4 and 20%. AGA and/or AEA were abnormal in 13 and normal in 5. Sensitivity was 80% for the three tests when used simultaneously and specificity was 100%. (ii) In the family study, 26 sera of asymptomatic first-degree relatives of IDDM patients were positive for at least one of the serological markers; 13 of them had villous atrophy. Systematic serological screening in IDDM allowed us to detect CD and evaluate the true incidence.