The neurofibromatosis type I gene encodes a protein, neurofibromin, which may function as a tumor suppressor gene product. Recent studies have demonstrated loss of neurofibromin in tumors from NF1 and non-NF1 patients, including neurofibrosarcomas, neuroblastomas and malignant melanomas. Since neurofibromin is expressed in the adrenal gland, six pheochromocytomas and one adrenal cortical tumor were examined for neurofibromin expression. In all seven tumors, no neurofibromin could be detected. Furthermore, loss of heterozygosity (LOH) analysis demonstrated that in one of the pheochromocytomas, reduction to homozygosity was observed for both 17p and 17q markers while the adrenal cortical tumor demonstrated LOH for only 17q markers. The frequent LOH surrounding the NF1 locus and lack of neurofibromin expression in these tumors suggest that NF1 gene mutations may contribute to the development of adrenal gland neoplasms in patients with NF1.