We found deletions of both alleles of the retinoblastoma gene (Rb) and the candidate Wilms' tumor gene (WT33) in a surgically resected human insulinoma by blot hybridization. Rearrangement of Krev-1, a transformation suppressor gene, was also detected in this case. The clinical and histological findings suggested that this tumor would be a benign insulinoma. The result suggests that the inactivation of multiple tumor suppressor genes could be involved even in the development of benign human tumors.