In a prospective study on a cohort of 7 year old children of low birth weight (under 2000 g at birth), we observed an increased prevalence of airway responsiveness to histamine compared with a reference population of unselected, local schoolchildren. The airway responsiveness to histamine was significantly related to a history of asthma in first degree relatives (natural parents and siblings) in both groups of children, but not to that of the mothers. There was no increase in the prevalence of maternal asthma, a family history of asthma, or airway responsiveness in the mothers of low birthweight children. We were unable to find evidence to support the hypothesis that maternal smooth muscle irritability (uterine and airway) has a causative role in the premature labour in the mothers and in subsequent bronchial hyper-responsiveness in their prematurely born children.