Three hundred asthmatic children aged 3 to 15 years, and 100 age-sex-neighborhood matched controls, were studied for environmental risk factors for asthma during 1986 to 1987. In 98% of the asthmatic patients, exposure to house dust aggravated the symptoms. In 97% of the cases, the symptoms were more severe at home; in more than 50%, the symptoms persisted throughout the year. Significantly more cigarettes were smoked in the homes of the allergic children than in the control homes. House dust samples from homes of 20 asthmatic and 20 nonasthmatic children were examined for the presence of house dust mites. Twelve species of mites were identified, of which Dermatophagoides pteronyssinus, the most prevalent (97%), was found in all samples examined. The average number of mites per gram dust was 447 in homes of children with asthma, and 399 in homes of controls. Although dust and mites seem to be the most important factors causing allergies in Gaza and most of the children are exposed to large numbers of mites in their homes, it appears that those who are genetically predisposed and who are exposed to environmental tobacco smoke are at greater risk for allergic symptoms.