Background: Intensive medicine is especially expensive and requires an efficient management. Aim: To measure the real costs of diseases treated in an intensive care unit and compare them with the costs assigned by the Chilean National Health Fund (FONASA) for 2008. Material and Methods: Retrospective review of 225 patients, representing 82% of discharges from an intensive care unit during 2008. Patients were classified according to their medical conditions as having sepsis, trauma, cardiovascular, respiratory or neurological diseases. Costs were calculated using the cost per activity system. Results: Trauma, sepsis and cardiovascular diseases had the greatest cost per inpatient day, corresponding to 294,779; 253,513 and 244,713 Chilean pesos, respectively. Seventy percent of costs correspond to human resources followed by complementary examinations, that represent up to 15% of costs. Patients with sepsis and cardiovascular diseases absorbed 28 and 26% of intensive care unit resources, respectively. Patients who died with these diseases absorbed 35 and 16% of resources, respectively. Conclusions: All diseases studied had significantly higher costs than those assigned by the National Health Fund.