Morphometry of 31 pancreatic islet cell tumors was examined to determine the value of this technique in assessing tumor behavior. Patients were followed for a mean period of 5.1 years (range, 1 month-14 years) after diagnosis. Initially 17 localized and nine metastatic tumors were studied. Discriminant analysis was carried out on these cases and identified nuclear/cytoplasmic ratio and number of nuclei/mm2 as the significant discriminatory features. These were combined to derive a classification rule which was capable of correctly identifying localized and metastatic tumors in 92% of cases. The classification rule was applied subsequently to an additional five test cases, all of which were classified successfully. The failure of increased nuclear size and pleomorphism to correlate with malignancy in these tumors was confirmed. Tumors which metastasized had significantly greater gross diameters than localized lesions, but overlap existed. Mitotic counts were not a helpful discriminatory feature. Morphometry may be useful in improving histologic assessment of pancreatic islet cell tumor behavior.