The aetiology of mouth ulcers is diverse and may include several types of trauma, systemic disease and infection. The size, depth, outline, base, aspect of the floor, pain, time of evolution and resolution of oral ulcers are discussed. Both past and present medical history, biopsy, blood tests and microbiological tests are also considered in order to assure precise identification. This paper reports a case of a lip ulcer caused by Klebsiella pneumoniae.