In order to study the familial occurrence of bronchial hypereactivity (BH) the parents of infants with BH were evaluated. 47 pairs of parents of infants aged 10-33 months were studied. All infants following spastic bronchitis developed BH. All parents had bronchial reactivity assessed. All answered a questionnaire from which the following data were retrieved: family history, personal history, environmental data and smoking habits. A similar analysis was carried out on a group of 19 pairs of parents forming a control group, derived randomly from hospital personnel. In parents of children with BH a higher incidence of BH was found in comparison with the control chi 2 = 11.52; p less than 0.01). In 14 subjects a positive familial history was found. This did not differ from the control. The smoking habit was seen more often in the study group that in the control (chi 2 = 9.82; p less than 0.01). These results allow us to state that familial factors play a role in bronchial hypereactivity in early infancy.