The authors carried out further studies to assess the significance of the findings of patients sensitive to cinnamic aldehyde in a toothpaste. Selected groups of patients in whom sensitivity to this might have been overlooked were patch-tested. With the exception of three patients, two with cosmetic sensitivity and one with hand eczema, the results were negative. The frequency of use of the original toothpaste was assessed among patients attending the clinic and this was followed by a circular letter to general practitioners and dentists inviting referral of patients with oral symptoms. As a result, five further cases, four of whom gave positive patch test reactions to cinnamic aldehyde, were discovered. The authors emphasize the importance of attempting to obtain accurate information as quickly as possible when 'new' or hitherto unsuspected allergens are implicated.