Trypsinogen activation can be quantified by measurement of released activation peptides (TAP assay). TAP assay in urine was performed on admission for 55 patients with acute pancreatitis. TAP concentration correlated with subsequent disease severity in 87%, whereas C-reactive protein concentration, and multifactorial scoring at 48 h, were correct in 55% and 84%. Sensitivity and specificity for TAP assay were 80% and 90%, for C-reactive protein 53% and 55%, and for multifactorial scoring at 48 h, 60% and 93%. Urine TAP assay distinguishes acute pancreatitis without trypsinogen activation from acute pancreatitis with trypsinogen activation, and helps to identify patients who will progress to the severe acute disease. Use of the assay should allow early intensive treatment of those who need it.