The hematologic status of 144 consecutive patients in the United Kingdom with oral disease (69 with recurrent aphthous stomatitis; 75 with other disorders) was examined in an attempt to resolve the controversy as to whether levels of hemoglobin, blood cell numbers and indices, and blood film are adequate in the screening of patients with aphthae. Though hemoglobin levels and red blood cell indices were normal in patients with recurrent aphthous stomatitis, there was still a small minority of patients with deficiencies of iron (low serum ferritin), folate (low red blood cell levels), or vitamin B12 (low serum levels) that would have remained undetected. Full hematologic screening of patients with recurrent aphthous stomatitis thus reveals latent deficiency states, at least in some parts of the world, and there are good reasons for elucidating the underlying cause and correcting these deficiencies, particularly vitamin B12 deficiency. Therapeutic studies are now required to establish the frequency with which deficiencies actually predispose to recurrent aphthous stomatitis.