We examined the utility of serum pancreatic lipase by radioimmunoassay as a diagnostic test for acute pancreatitis and its correlation with serum total amylase, pancreatic isoamylase, and lipase activity. Data were analyzed on 11 patients with documented acute pancreatitis, three groups of patients (N = 104) with nongastrointestinal, gastrointestinal, and renal diseases, and 30 healthy controls. Patients with acute pancreatitis had significantly (P less than 0.01) higher mean serum lipase by radioimmunoassay than all other groups. Using a serum lipase of 112 ng/ml as a cutoff point in all patients, the test was 91% sensitive and 96% specific for the diagnosis of acute pancreatitis. The correlation coefficients of serum lipase by radioimmunoassay with respect to total amylase, pancreatic isoamylase, and lipase activity were 0.86, 0.98, and 0.79, respectively.