Flea allergy dermatitis (FAD) is the most common veterinary dermatologic condition in the world. It begins with the bite of a flea. The saliva of the flea contains amino acids, aromatic compounds, fluorescent materials, polypeptides, and phosphorus. (9) In the dog the antigenic substances carry a molecular weight of 18,000 to 45,000 daltons with the major allergen of MW 30,000 to 32,000 daltons. (10) The main thing that dog owners notice is scratching, chewing, licking, biting, and other signs of pruritus. Sixty one percent of flea allergic dogs develop clinical signs between 1 and 3 years of age. (11) As animals age, with continued exposure to fleas, the degree of hypersensitivity may wane. FAD is uncommon in dogs less than 6 months of age. Patients usually have papules, crusts, salivary stains, excoriations, and erythema in a wedge-shaped pattern over the lumbosacral region, caudal thighs, proximal tail, ventral abdomen, and around the umbilicus. With chronic itching the areas become alopecic, lichenified, and hyperpigmented and the dog will develop an odor related to secondary infections with Staphylococcus intermedius and Malassezia pachydermatis. A diagnosis of FAD is based on the age of onset of the pruritus, the distribution of the pruritus and clinical signs, and the observation of fleas and/or flea feces. Many dogs who are allergic to the bite of a flea have very few fleas on them at any time because their excessive grooming activity removes the fleas. Some of those patients will have recurrent tapeworm (Diplydium caninum) infestations from ingestion of the fleas. The diagnosis of FAD can be confirmed with an intradermal skin test with flea antigen.