BACKGROUND: Patients with coeliac disease living on a gluten-free diet show vitamin deficiency and reduced subjective health status. AIM: To study the biochemical and clinical effects of B vitamin supplementation in adults with longstanding coeliac disease. METHODS: In a double blind placebo controlled multicentre trial, 65 coeliac patients (61% women) aged 45-64 years on a strict gluten-free diet for several years were randomized to a daily dose of 0.8 mg folic acid,0.5 mg cyanocobalamin and 3 mg pyridoxine or placebo for 6 months. The outcome measures were psychological general well-being (PGWB) and the plasma total homocysteine (tHcy) level, marker of B vitamin status. RESULTS: Fifty-seven patients (88%) completed the trial. The tHcy level was baseline median 11.7 micromol/L (7.4-23.0), significantly higher than in matched population controls [10.2 micromol/L (6.7-22.6) (P < 0.01)]. Following vitamin supplementation, tHcy dropped a median of 34% (P < 0.001), accompanied by significant improvement in well-being (P < 0.01), notably Anxiety (P < 0.05) and Depressed Mood (P < 0.05) for patients with poor well-being. CONCLUSIONS: Adults with longstanding coeliac disease taking extra B vitamins for 6 months showed normalized tHcy and significant improvement in general well-being, suggesting that B vitamins should be considered in people advised to follow a gluten-free diet.