Leptin is secreted into the circulation and communicates the peripheral nutritional status to specific hypothalamic centers. Recent studies suggest that leptin may be involved in the acute response to stress, and that its interaction with the hypothalamo-pituitary-adrenal axis and the inflammatory cytokine system may be of clinical importance. Since these systems are activated during acute myocardial infarction (AMI), we studied leptin and cortisol levels during hospitalization in 30 consecutive patients admitted for AMI. The results show that leptin reached its peak on the second day of hospitalization, with a 2-fold increase from its baseline level on admission (p < 0.02). On day 3, leptin levels declined, and were 46%, 9%, and 6% above baseline on days 3, 4 and 5, respectively. The mean cortisol level was elevated on day 1 and decreased toward normal levels thereafter (p < 0.001). The cortisol level did not correlate with leptin concentration throughout the study. These findings suggest that leptin may have a role in the metabolic changes taking place during the first days after an AMI.