Clinical studies were conducted to evaluate the role of the house dust mite. Dermatophagoides pteronyssinus, as an allergen in children's asthma in Japan. Skin testing and Prausnitz-Küstner type skin testing showed good correlations between the allergenicity of house dust and the mite. A positive Prausnitz-Küstner type skin reaction to house dust was neutralized by a mite extract. Radioallergosorbent test showed good correlation between house dust and the mite (r=0.687). The mite exhibited in vitro inhibition of radioallergosorbent test to house dust. The inhalation provocation test resulted in an acute respiratory attack in one of the patients. These results suggest that the house dust mite is an important allergen in asthmatic children in Japan.