A series of 130 consecutive outpatients with recurrent aphthous stomatitis were screened at the oral medicine department, Glasgow Dental Hospital, for deficienciesin vitamin b12, folic acid, and iron. In 23 patients (17.7%) such deficiencies werefound; five were deficient in vitamin B12, seven in folic acid, and 15 in iron. Four had more than one deficiency. Out of 130 controls matched for age and sex 11 (8.5%) were found to have deficiencies. The 23 deficient patients with recurrent aphthaewere treated with specific replacement therapy, and all 130 patients were followed up for at least one year. Of the 23 patients on replacement therapy 15 showed complete remission of ulceration and eight definite improvement. Of the 107 patientswith no deficiency receiving local symptomatic treatment only 33 had a remission or wereimproved. This difference was significant (P less than 0.001). Most patients withproved vitamin B12 or folic acid deficiency improved rapidly on replacement therapy;those with iron deficiency showed a less dramatic response. The 23 deficient patientswere further investigated to determine the cause of their deficiencies and detect the presence of any associated conditions. Four were found to have Addisonian perniciousanaemia. Seven had a malabsorption syndrome, which in five proved to be a gluten-induced enteropathy. In addition, there were single patients with idiopathic proctocolitis, diverticular disease of the colon, regional enterocolitis, and adenocarcinoma of thecaecum. We suggest that the high incidence of deficiencies found in this series andthe good response to replacement therapy shows the need for haematological screening of such patients.