The possible involvement ofCytomegalovirus(CMV) and Helicobacter pylori (HP) in oral mucosal ulcers is suggested by their role in the development of ulceration at other mucosal sites of the gastrointestinal tract. A series of 29 incisional biopsies from 29 consecutive and apparently immunocompetent patients attending the clinic for oral ulceration were examined by routine histopathology as well as byin situhybridisation (ISH) with biotinylated CMV and HP DNA probes. In 14/29 biopsies, Giemsa staining disclosed spiral bacteria. Six (20.7%) of these 14 Giemsa‐positive samples showed HP DNA on ISH and 3 ulcers (10.3%) contained CMV DNA. In none of the specimens were CMV and HP detected simultaneously. Two of the ulcers containing CMV DNA were found on the labial mucosa and one on the posterior palatal mucosa, whereas all HP DNA‐positive ulcers were located on the buccal mucosa. The results indicate that CMV and HP DNA can be found in separate oral mucosal ulcers in apparently immunocompetent adu