Objective: As hormones are strongly associated with mortality in critically ill patients, we investigated whether mortality prediction based on the Acute Physiology and Chronic Health Evaluation (APACHE) is improved by combining this score with hormone measurements. Design and setting: Intensive care units in three hospitals Patients and participants: 113 patients admitted to. Measurements: Within the first hour after ICU admission we measured total triiodothyronine, total thyroxine, free thyroxine, thyrotropin, cortisol, growth hormone, dehydroepiandrosterone, and prolactin levels and administered the APACHE. Patients were followed until they died or were discharged from the ICU. Results: The best logistic regression model for ICU mortality included the APACHE score and thyroid-stimulating hormone and triiodothyronine levels. This model had an area under the receiver operating characteristic curve of 0.88, significantly higher than the APACHE score alone with 0.75. The model with hormone levels and APACHE score was also significantly better calibrated than the model with only the APACHE score. Conclusions: The addition of thyroid hormones to the APACHE score improves the prediction of mortality for ICU patients © Springer-Verlag 2005.