We had a two-month-old girl with severe dermatitis since birth. Her serum RAST to HD, Df and Dp were 1.06, 0.03 and 0.01 Ua/ml respectively. A Yorkshire terrier were kept at her mother's parents' home where the patient had lived for a month since birth. Her eczema, which became markedly aggravated whenever she visited there, improved after the elimination of the dog. We investigated the relationship between keeping dogs and infantile atopic dermatitis. We studied 368 patients under the age of two years (211 boys and 157 girls). Skin symptoms were graded globally mild, moderate or severe. Total serum IgE and specific antibody titer to dog dander were measured. We asked them whether they kept dogs and specifically, where they kept dogs, outdoor, indoor, in their own house, or in their grandparents' house. 197 patients had no contact with dogs, 90 patients kept dogs outdoor and 81 patients did indoor. The positive rate of RAST (> or = 0.7 Ua/ml) to dog dander was 6.1%, 17.8% and 46.9% respectively in these three groups. There were strong statistical differences between three groups. On the other hand, among the 81 patients who kept indoor, the RAST positive rates were almost same regarding where the dogs were kept, in their own house or their grandparents' house. Interestingly this difference happens only with patients under the age of 3 months. Patients older than 4 months showed no significant differences in the positive RAST rates, whether they kept dogs indoor or outdoor. This suggests the sensitization occurs before the age of 3 months. Speaking of symptoms, patients who kept dogs indoor showed significantly more severe symptoms than patients who had no contact with dogs and patients who kept dogs outdoor. There was no significant difference between the symptoms of patients who had no contact with dogs and those of patients who kept dogs outdoor. This implies the patient's symptom will improve only by moving the dog out of the house.