Sequential daily measurements of plasma parathyroid hormone (PTH) have been performed in 89 patients with acute pancreatitis. A total of 439 PTH assays was obtained during 98 episodes of the disease. Three main patterns of PTH response were found. These responses were correlated with severity of disease graded by objective criteria and also to corrected serum calcium levels. The first type of PTH response was characterized by significantly elevated PTH levels soon after hospitalization with a subsequent decrease in levels to within the normal range by the third or fourth day of illness. This type of response was specifically associated with transient severe hypocalcaemia (corrected calcium less than 2.0 mmol/l). It was also associated with the most severe forms of disease and 6 of the 7 deaths. The second type of PTH response revealed initial PTH values in the upper level of the normal range (400--600 ng/l) while persistently low PTH levels were characteristic of the third type of response. Persistently low PTH levels were associated with normocalcaemia, and no patient in this group died. None of a group of 14 control patients exhibited the type 1 PTH response. An effective PTH response to an unidentified hypocalcaemic stimulus results in satisfactory calcium homeostasis in most patients with acute pancreatitis.