A population survey of 1000 7 year old children found a significant excess of wheeze among children whose homes were reported to be mouldy (odds ratio 3.70, 95% confidence limits 2.22, 6.15). The airborne mould flora was quantified by repeated volumetric sampling during the winter in three rooms of the homes of 88 children. All of these had previously completed spirometric tests before and after a six minute free running exercise challenge. Total airborne mould counts varied from 0 to 41,000 colony forming units (CFU)/m3, but were generally in the range 50-1500 CFU/m3, much lower than the concentrations found outdoors in summer. The principal types of fungi identified are all known to be common out of doors, and most were found on at least one occasion in most of the homes. Median and geometric mean total mould counts were not related to reports of visible mould in the home, or to a history of wheeze in the index child. The heterogeneous group of non-sporing fungi (mycelia sterilia) were the only airborne fungi present at significantly higher concentrations in the homes of wheezy children (geometric mean 2.1 v 0.7 CFU/m3. A non-significant increase in total mould counts was observed in the homes of children with a 10% or greater decline in FEV1 after exercise (geometric mean 354 v 253 CFU/m3). Questionnaire reports of mould in the home may be a poor indicator of exposure to airborne spores. The total burden of inhaled mould spores from indoor sources is probably not an important determinant of wheeze among children in the general population. Although the association with mycelia sterilia could be a chance finding, these non-sporing isolates may include a potent source of allergen.