Intraoperative fine needle aspiration biopsy (NAB) of undiagnosed pancreatic masses was studied in 166 patients over a 17-year period. The cytologic diagnoses were correlated with histologic specimens, autopsy results, or clinical follow-up (benign disease was documented if the patient was alive without malignancy at least 2 years after laparotomy). Aspirates were interpreted as benign, suspicious, malignant, or unsatisfactory. Malignant disease was the final diagnosis in 109 patients; the cytology was concordant in 101 and was interpreted as suspicious in four. Four patients with benign cytology later proved to have malignant disease--a false-negative rate of 2.5 per cent. A total of 57 patients had benign disease; 51 of these had benign cytology. The remaining patients had "unsatisfactory" cytology reports. A 93 per cent sensitivity, 100 per cent specificity, and 0 per cent complication rate are reported. There were no false-positive cytology reports. Complications are rare and represent case reports, thus, additional sampling is at minimal risk. Intraoperative pancreatic NAB is a safe, easy, more accurate biopsy technique than historical wedge or core needle biopsies. It is the biopsy method of choice for pancreatic masses found at laparotomy.