Coeliac disease has been reported to occur in 2-5 per cent of insulin-dependent diabetic patients (IDDM). Suitable non-invasive screening tests would allow identification of these patients. The aim of this study was to determine the value of the red cell distribution width (RDW) in detecting unrecognised coeliac disease in insulin-dependent diabetic patients (IDDM). All patients (n = 208) attending the Diabetic out-patient clinics at 2 adjacent centres who had a full blood picture and RDW carried out in the past 18 months were included. IDDM patients with an elevated RDW were identified and their charts were reviewed to determine if they had symptoms or laboratory abnormalities compatible with coeliac disease. They were invited to attend for serological screening. Ninety-five of 208 patients had an elevated RDW of whom 66 had non-insulin dependent diabetes mellitus and 29 had IDDM. Two of the 29 IDDM patients had died in the interim period. Six of the remaining 27 IDDM patients had previously been tested for serological markers associated with coeliac disease, of whom 1 had a positive antigliadin antibody titre (IgA-AGA 199 EU) and normal duodenal biopsy. Eighteen of the remaining 21 patients with IDDM consented to serological testing of whom only 1 had a positive titre of antiendomysial antibody (IgA-EMA) and villous atrophy. Although the RDW is known from previous studies to be a sensitive predictor for coeliac disease, this study has demonstrated its poor specificity in predicting IDDM patients who may have coeliac disease. The RDW is not recommended as a screening test for coeliac disease in patients with IDDM.