On 13 September 1981, a 51-year-old seven-story building within our hospital complex was demolished by explosives. The concern that this event might release large numbers of thermotolerant fungi (TF), potentially hazardous to immunosuppressed patients, led us to seal hospital windows and doors. The air-handling systems were also manipulated. Concentrations of airborne TF, especially Aspergillus fumigatus, were determined before and after demolition, using Andersen and Cassella air samplers with inhibitory mold agar plates. Two outdoor and two hospital ward locations were sampled. The plates were incubated at 37°C; the CFU per cubic meter were counted at 72 h. The outdoor concentration of TF increased at one site by an average of 1.8 log10 (102 to 105) over the predemolition level. A. fumigatus increased 3.3 log10 (100 to 104) at the other outdoor site. The indoor TF concentrations increased about 1 log10 (101 to 102) after demolition. Counts on the hospital wards were not remarkable when compared with previous surveillance air sampling. Protective measures apparently minimized the infiltration of TF during explosive demolition.