We investigated peripheral lymphocyte subsets in 34 consecutive acute pancreatitis patients (21 males, 13 females; mean age, 57 years; range, 16-85 years) studied within 48 h of pain onset and for 5 consecutive days to understand better the immunological response during the course of the disease. The diagnosis was based on characteristic abdominal pain associated with a twofold increase in serum lipase and confirmed by imaging techniques in all patients. Acute pancreatitis was of biliary origin in 25 patients, due to alcohol abuse in 5, due to pancreas divisum in 1, and of unknown origin in 3. Fifteen patients had severe illness and 19 had mild disease. In all patients, total lymphocyte and lymphocyte subset counts were carried out on admission, as well as on the third and fifth day of hospitalization, using a flow cytometric analysis. Twenty-three patients (13 with severe illness and 10 with mild disease) also had a repeat count 1 month after recovery. Twenty-five healthy subjects and 27 patients with nonpancreatic acute abdomen comparable for sex and age were studied as controls. On the first day of the study, the leukocyte number was significantly higher in patients with acute pancreatitis and in those with nonpancreatic acute abdomen with respect to healthy subjects, whereas the number of total and CD4+, CD8+, CD3+ DR-, and CD3- DR+ lymphocytes was significantly lower in acute pancreatitis patients than in healthy subjects or in patients with nonpancreatic acute abdomen. These subject counts persisted on the third and fifth days of the study.(ABSTRACT TRUNCATED AT 250 WORDS)