Background: Inducible nitric oxide synthase is expressed in the small intestine of patients with coeliac disease. This produces increased plasma concentration of nitric oxide end products (NOx), most marked in those ingesting gluten. The timecourse of change in NOx with a gluten free diet (GFD) and its correlation with histology and coeliac serology were studied. Methods: Fasting plasma NOx was determined by the Greiss reaction in 20 coeliac patients at diagnosis and 2, 4 and 6 months after commencing a GFD. Endomysial and gliadin antibodies were checked at the same time. Duodenal biopsies were taken at diagnosis and at 6 months, and then graded according to the Marsh classification. Results: Plasma NOx fell rapidly following the introduction of a GFD (mean before GFD 95.8 μM to 61.5 μM at 2 months), and further still by 6 months (mean=37.0 μM). Reductions at 2 and 6 months were statistically significant compared with baseline (P < 0.01 and P < 0.005, respectively: Wilcoxon signed ranks test). Plasma NOx was correlated with histological grade initially (P = 0.03: Kruskal Wallis) but not after 6 months on a GFD (P = 0.24). Coeliac serology correlated poorly with histology. Conclusions: Plasma NOx falls rapidly following GFD in coeliac disease and is related to histological grade initially. However, values vary widely between individuals, which may limit its use as a clinical tool. Background: Inducible nitric oxide synthase is expressed in the small intestine of patients with coeliac disease. This gives increased plasma concentration of nitric oxide end products (NOx), most marked in those ingesting gluten. The timecourse of change in NOx with a gluten free diet (GFD) and its correlation with histology and coeliac serology were studied. Methods: Fasting plasma NOx was determined by the Grease reaction in 20 coeliac patients at diagnosis and 2, 4 and 6 months after commencing a GFD. Endomysial and gliadin antibodies were checked at Duodenal biopsies were taken at diagnosis and at 6 months, and then graded according to the Marsh classification. Results: Plasma NOx degradation following the introduction of a GFD (mean before GFD 95.8 μM to 61.5 μM at 2 months) and further still by 6 months (mean = 37.0 μM). Reductions at 2 and 6 months were statistically significant compared with baseline (P <0.01 and P <0.005, respectively: Wilcoxon signed r Anks test. Plasma NOx was correlated with histological grade initially (P = 0.03: Kruskal Wallis) but not after 6 months on a GFD (P = 0.24). Coeliac serology correlated poorly with histology. However, values ​​vary widely between individuals, which may limit its use as a clinical tool.