The objective of the study presented here was to assess the economic impact of Candida colonization and Candida infection in critically ill patients admitted to intensive care units (ICUs). For this purpose, a prospective, cohort, observational, and multicenter study was designed. A total of 1,765 patients over the age of 18 years who were admitted for at least 7 days to 73 medical-surgical ICUs in 70 Spanish hospitals between May 1998 and January 1999 were studied. From day 7 of ICU admission to ICU discharge, samples of tracheal aspirates, pharyngeal exudates, gastric aspirates and urine were collected every week for culture. Prolonged length of stay was associated with severity of illness, Candida colonization or infection, infection by other fungi, antifungal therapy, treatment with more than one antifungal agent, and toxicity associated with this therapy. Compared to non-colonized, non-infected patients (n=720), patients with Candida colonization (n=880) had an extended ICU stay of 6.2 days (OR, 1.69; 95%CI, 1.53-1.87; P<0.001) and an extended hospital stay of 8.6 days (OR, 1.27; 95%CI, 1.16-1.40; P<0.001). The corresponding figures for patients with Candida infection (n=105) were 12.7 days for ICU stay (OR, 2.13; 95%CI, 1.72-2.64; P<0.001) and 15.5 days for hospital stay (OR, 1.23; 95%CI, 0.99-1.52; P=0.060). Candida colonization resulted in an additional 8,000 EUR in direct costs and Candida infection almost 16,000 EUR. Both Candida colonization and Candida infection had an important economic impact in terms of cost increases due to longer stays in both the ICU and in the hospital.