The most common inherited syndrome in man predisposing to neoplasia is neurofibromatosis-1 (von Recklinghausen disease) (NF1). We investigated the hypothesis that affected individuals carry a single inactive allele at the NF1 locus in the germline and that a tumor arises from a cell in a susceptible tissue in which the remaining normal allele has been lost or inactivated. DNA from tumor and nontumor tissue from 27 NF1 patients was analyzed with three markers closely linked to the NF1 locus and two additional markers from chromosome 17. No loss of heterozygosity was observed in neurofibromas, plexiform or not. For other tumor types analyzed, seven of 14 showed a loss. A loss of heterozygosity was observed in six of 11 of the malignant peripheral nerve tumors analyzed. Of the seven malignancies demonstrating a loss, five involved a neurofibrosarcoma. These findings suggest that the pathogenesis of neurofibrosarcoma in NF1 involves a deficiency of the NF1 gene product. In any given patient, loss of heterozygosity was detected at some marker loci but not others. Thus the mutations demonstrated in these tumors comprise a set of overlapping mutations, which may facilitate more precise localization of the NF1 gene.