A new radioimmunoassay to serum trypsinogen (Cis Trypsik) was tested in several patient populations. A low serum trypsinogen level (less than 10 ng/ml) was found in 69.2% of 13 patients with chronic pancreatic insufficiency (CPI), in 100% of 10 patients with 95-100% pancreatectomy but only in 14% of 14 patients with cancer of the pancreas. A low trypsinogen level was not found in any of 68 control subjects or 10 patients with nonpancreatic steatorrhea. Nine patients with CPI or 95% pancreatectomy were retested a mean of six months after initial testing. Four of these nine (44.4%) had a significant variation in serum trypsinogen which would have led to a different diagnostic interpretation (two went from low to normal levels and two from normal to low levels). A mixed meal had little effect on serum trypsinogen levels in five of six patients with CPI, and pancreatic enzyme replacement therapy had no consistent effect on the serum trypsinogen level in seven patients with CPI or 95% pancreatectomy. It is speculated that minor subclinical episodes of focal pancreatitis may effect the serum trypsinogen level. Although there can be considerable variability using this assay, it still offers important clinical utility. A low trypsinogen level points to a chronic pancreatic process with excellent specificity. A normal trypsinogen level is of no help and should be repeated if clinical suspicion of chronic pancreatitis remains high.