In the Zollinger-Ellison syndrome, fundic argyrophil carcinoid tumors have been described only in the small genetically defined subgroup of patients who have the multiple endocrine neoplasia type 1 syndrome (MEN-1). Allelic losses on 11q13, on which MEN-1 gene has been localized, have been noted in parathyroid and pancreatic tumors of patients with MEN-1, suggesting that the MEN-1 gene could act as a recessive tumor suppressor gene. One fundic argyrophil carcinoid tumor from a patient with the Zollinger-Ellison syndrome and MEN-1 was studied. Loss of heterozygosity in the tumor DNA at loci close to MEN-1 locus was looked for using Southern technique with six DNA probes. Segregation of alleles was examined in relatives. In the tumor DNA, we found the loss of one allele with PYGM, the closest probe to the MEN-1 locus. The allele lost in the tumor had been transmitted by the unaffected parent. This suggests that in patients with the Zollinger-Ellison syndrome and MEN-1, the promotion of fundic argyrophil carcinoid tumors results from the inactivation of the two copies of MEN-1 gene and that fundic argyrophil carcinoid tumors may be included in the spectrum of MEN-1-related tumors.