Over a 5-month period, six patients in a general intensive therapy unit became colonized by Aspergillus species including Aspergillus fumigatus, and invasive infection occurred in at least two of them. Broncho-alveolar lavage was unhelpful in discriminating between infection and colonization. The source of infection was presumed to be disturbance of an accumulation of spores in fibrous insulation material above the perforated metal ceiling. Patients in such units without clearly identifiable defects of defence against infection may be at risk from aspergillosis. The risk can be reduced by improved hospital design, satisfactory ventilation and thorough regular cleaning of environmental surfaces.