A sequential sampling study of fungal airspores was carried out with the Andersen sampler inside and outside the homes of eight asthmatic, mould-sensitive patients. The aim of the study was to find a possible relationship between variations in airborne colony forming units (CFU) and the occurrence of increased pulmonary complaints (IPC). Great variations in fungal prevalence occurred throughout the year, the lowest average daily concentration being 125, and the highest, 1425 CFU/m3/sample during the whole survey. Penicillium dominated the catch in seven of the eight environments. Other frequently sampled moulds were Cladosporium, yeasts, Aspergillus, and Botrytis. Sixty-six percent of IPC were noted from July through November, during which time 56% of all fungal peaks occurred. A significant difference was found between peak flow (PF) values on days with the highest mould numbers compared with PF values on days with low counts. This relation was not found for PF and indoor mould counts. No particular fungal genus was responsible for IPC in general, but in a given patient the occurrence of a special mould type may be related to obstructive reactions. It is concluded that a possible relationship exists between the prevalence of airborne fungi and IPC. Definite proof of a direct bronchial obstructive effect, however, might only be provided by means of inhalation-provocation studies with the sampled fungi.