The clinical significance of a burn depends on the percentage of total body involved and the depth of the burn. Hence a noninvasive method that is able to evaluate burn depth would be of great help in clinical evaluation. To this end, photoacoustic microscopy is used to determine the depth of acute thermal burns by imaging the total hemoglobin concentration in the blood that accumulates along the boundaries of injuries as a result of thermal damage to the vasculature. We induce acute thermal burns in vivo on pig skin with cautery. Photoacoustic images of the burns are acquired after skin excision. In a burn treated at 175 degrees C for 20 s, the maximum imaged burn depth is 1.73+/-0.07 mm. In burns treated at 150 degrees C for 5, 10, 20, and 30 s, respectively, the trend of increasing maximum burn depth with longer thermal exposure is demonstrated.