Linkage data using the markers DXS51, F9, DXS15, and DXS52 are presented from 14 pedigrees segregating with the fragile X. Cytogenetic and DNA data were combined by two- or three-point linkage analysis for estimation of lod scores and carrier probabilities in potential carriers. Recombination frequencies (θ) corresponding to maximum z scores (z) were obtained for DXS51 (z = 3.45, θ = 0.0), DXS15 (z = 0.40, θ = 0.06), F9 (z = 3.15, θ = 0.09), and DXS52 (z = 3.60, θ = 0.11) with the fragile X. Considerable alterations to carrier probabilities occurred in some cases, especially when flanking markers were informative. The chance of mentally impaired offspring was reduced to 1% for five of eight women with prior carrier probabilities of 32%. Three pedigrees were identified in which mutation had possibly occurred. An alternative explanation for two of these was inheritance of the fragile X from normal males and for the other inheritance from a clinically normal woman. Probabilities were computed for each of these alternatives.