Pattern of endocrine changes in moderate to severely ill patients in a medical intensive care unit, correlation with the severity of illness and whether these changes can predict outcome of the critically ill patients were evaluated and studied in 80 patients admitted with acute physiology and chronic health evaluation (APACHE) II score >10 and without any pre-existing endocrinopathies or on drugs likely to affect the endocrine axis. Adrenal insufficiency was present in 45%, and mortality was higher in those with lower (<15 microg/dl) and higher (>30 microg/dl) serum cortisol. Sick euthyroid syndrome was detected in 80%, and those with low mean T3 (<0.6 ng/ml), free T4 (<0.89 ng/dl) and total T4 (<4 microg/dl) and had increased mortality. Hypotestosteronaemia was found in 92% of men and was significantly associated with severity of illness in men. Though prolactin is the first hormone to be elevated, there was no correlation between prolactin and severity of illness or mortality.