DNA nuclear content and PCNA index (proportion of PCNA reactive cells) have been studied by flow cytometry in eight pancreatic lesions producing hyperinsulinemic hypoglycemia to assess DNA ploidy and tumoral growth fraction. Adult nesidioblastosis had a diploid DNA index, two adenomas were near diploid, and four of the adenomas and the carcinoma were aneuploid. The median value of PCNA index (6.056% ± 6.76) was significantly correlated (p <0.05) with the mean tumor diameter (2.43 cm ± 1.96). Tumors with a PCNA index <6.056% showed a diameter less than the mean tumoral diameter and a benign clinical course. Tumors with PCNA index <6.056% generally displayed a diameter bigger than the mean tumoral diameter, being associated with lymph node metastases in one case. The authors conclude that nuclear DNA and PCNA index cytometric studies are useful parameters to assess the biological behaviour of pancreatic lesions producing hyperinsulinemic hypoglycemia.