We present a case of recurrent herpes labialis treated with acyclovir. The patient attended within 3 h of the onset of prodrome of the condition, when he appeared clinically normal. Electronic infra-red thermography of the face and lips was undertaken and demonstrated a significant sub-clinical local elevation in temperature of 1.7 degrees C at the site where prodromal symptoms were perceived. Treatment with topical acyclovir cream five times daily was commenced immediately and aborted the developing lesion. Two days after presentation the patient was thermographically normal. To our knowledge this is the first report documenting the abortion of herpes labialis treated with acyclovir where the prodrome was confirmed thermographically.