The infiltration of leukocytes has been linked to the pathophysiology of complicated or severe pancreatitis. We have tested the ability of leukocyte scintigraphy using technetium-99m-hexamethyl propylene amine oxine (HM-PAO) as label to demonstrate the localization of leukocytes in the pancreas during acute pancreatitis. Twenty-eight patients with acute pancreatitis (eight with biliary, 13 with alcoholic, and seven with unknown origin) were studied with leukocyte scintigraphy using planar imaging and single photon emission computed tomography (SPECT). Fourteen patients had a mild (group I), II a severe (group II), and three a lethal outcome (group III) of pancreatitis. All patients of group III, six of group II, and two of group I had a positive leukocyte scan. Thus, the sensitivity of leukocyte scintigraphy for the detection of a lethal course of acute pancreatitis was 100%, of a severe course 54%, and of a severe or lethal course 64%. The specificity of a negative scan for a mild pancreatitis was 86%. Comparison of the results of leukocyte scintigraphy with those of contrast enhanced CT showed that six of eight patients with pancreatic necrosis in CT had a positive leukocyte scan, but only five of 20 patients without detectable pancreatic necrosis in CT. In summary, leukocyte infiltration into the pancreas during pancreatitis can be demonstrated by noninvasive leukocyte scintigraphy using technetium-99m-HM-PAO as label. A correlation between the severity of the disease and leukocyte infiltration exists.