Invasive aspergillosis is a rare disease in intensive care unit (ICU) patients and carries a poor prognosis. The aim of the present study was to determine the attributable mortality due to invasive aspergillosis in critically ill patients. In a retrospective, matched cohort study (July 1997-December 1999), 37 ICU patients with invasive aspergillosis were identified together with 74 control patients. Matching of control (1:2) patients was based on the acute physiology and chronic health evaluation (APACHE) II classification: an equal APACHE II score (+/-1 point) and diagnostic category. This matching procedure results in an equal expected in-hospital mortality for cases and controls. Additionally, control patients were required to have an ICU stay equivalent to or longer than the case before the first culture positive for Aspergillus spp. Patients with invasive aspergillosis were more likely to experience acute renal failure (43.2% versus 20.5%; P = 0.020). They also had a longer ICU stay (median: 13 days versus seven days; P < 0.001) as well as a more extended period of mechanical ventilator dependency (median: 13 days versus four days; P < 0.001). Hospital mortalities for cases and controls were 75.7% versus 56.8%, respectively (P=0.051). The attributable mortality was 18.9% (95% CI: 1.1-36.7). A multivariate survival analysis showed invasive aspergillosis [hazard ratio (HR): 1.9, 95% CI: 1.2-3.0; P = 0.004] and acute respiratory failure (HR: 6.5, 95%: 1.4-29.3; P < 0.016) to be independently associated with in-hospital mortality. In conclusion, it was found that invasive aspergillosis in ICU patients carries a significant attributable mortality of 18.9%. In a multivariate analysis, adjusting for other co-morbidity factors, invasive aspergillosis was recognized as an independent predictor of mortality.