Relationships among genetic markers in the region of the neurofibromatosis type 1 (NF1) gene on chromosome 17 were investigated by linkage studies in a large sample set of affected families and in a panel of 58 normal families. A new marker, pHHH202 (D17S33), was included along with two markers known to be closely linked to NF. The maximum likelihood estimate of the recombination rate between the pHHH202 and NF1 loci was found to be O. Multilocus analysis suggested the following marker order: pA10-41-(p3-6, pHHH202); the NF1 gene fell with equal likelihood between either pA10-41-p3-6 or p3-6-pHHH202. The odds against NF1 being outside this cluster of tightly linked markers were greater than 15:1.