Sixteen patients developed a variety of oral lesions following a change in the toothpaste they used. Mucosal biopsy demonstrated features consistent with application of a topical medicament and patch testing, towards the constituents of the toothpastes, indicated the flavouring agent cinnamonaldehyde as being the likely responsible agent. Avoidance of the implicated toothpastes resulted in a considerable improvement in clinical signs and symptoms, whereas rechallenge in ten patients resulted in recurrence of symptoms in eight patients. It is concluded that sensitivity to the cinnamonaldehyde constituent of certain toothpastes, although uncommon, should be considered as a possible aetiological factor in patients presenting with non-specific oral changes.