During the course of extensive building activity at Guy's Hospital, London, air sampling was carried out weekly, for one year, to monitor the frequency of spores of Aspergillus spp. in both the hospital grounds and a number of defined ward areas. Nasal swabs were taken from patients on a selected ward to assess the nasal carriage of Aspergillus. The weather conditions were monitored at the time of sampling. The predominant species was Aspergillus fumigatus which occurred at a low frequency throughout the year, with little evidence of seasonal variation bar a peak in March. Few differences were found between internal and external spore counts. Localized building activity in a ward area and a laboratory area did not result in higher internal levels. Six percent of the nasal swabs were positive for A. fumigatus, though none of these patients became infected.