We evaluated the behavior of sera interleukin p70 (IL-12p70) and IL-12p40 in patients with acute pancreatitis. Twenty-three acute pancreatitis patients were studied (12 had severe pancreatitis and 11 had mild pancreatitis). Twenty healthy subjects were studied as controls. Serum concentrations of total IL-12, IL-12p70, IL-12p40, and IL-6 were determined in all subjects on admission to the hospital; in patients with acute pancreatitis, the serum levels of these molecules also were determined for the 5 days after admission. Acute pancreatitis patients had serum concentrations of total IL-12, IL-12p40, and IL-6 significantly higher (p < 0.05) than those of the healthy subjects from the first to the sixth day of the study; serum concentrations of IL-12p70 were significantly higher in acute pancreatitis patients (p < 0.02) than in the healthy subjects on the first of the disease and significantly lower (p < 0.05) on the second, third, and fourth days of illness. Reduction of IL-12p70 in acute pancreatitis patients was not due to a failure of the monocytes because the production of IL-6 was increased. Increased amounts of IL-12p40 in acute pancreatitis patients may be responsible for their increased susceptibility to infection.