A series of 330 patients with recurrent aphthae was screened for deficiencies of iron, folate and vitamin B12. In 47 patients (14.2%) such deficiencies were found; 23 were deficient in iron, seven in folic acid, six in vitamin B12 and in addition 11 patients had combined deficiencies. Clinical examination of the aphthae was not helpful in identifying individual patients with a nutritional deficiency although patients with an associated glossitis or angular cheilitis were more likely to suffer from such deficiencies. Screening of the patients by examination of their peripheral blood alone (estimation of haemoglobin and absolute values, and blood film examination) detected only a proportion of those with deficiencies of iron or folic acid, although in this series such screening was able to identify the small number of cases with vitamin B12 deficiency. The 33 patients with a proven nutritional deficiency who were available for follow-up showed a favourable response to corrective therapy; 23 showed a complete remission of ulcers, 11 were improved and five were not helped. The significance of these findings is discussed. It is suggested that the results indicate the need for full haematological screening of all patients with recurrent aphthae.