A gene for facioscapulohumeral muscular dystrophy (FSHD) has been linked to chromosome 4q35 in families with the disease. We have used recently characterized p13E-11/D4S809 probes that map near or within the FSHD gene to investigate eight sporadic cases of FSHD whose parents showed no signs of disease. Probe p13E-11/D4S809 detected novel DNA fragments in seven of the eight sporadic FSHD individuals and not in the parents, substantiating the clinical diagnosis. Two sisters with FSHD whose parents were clinically normal had a novel DNA fragment suggestive of germline mosaicism. Probe p13E-ll/D4S809 is potentially helpful in genetic counseling. However, because this probe may also detect a locus unlinked to chromosome 4, because of possible genetic heterogeneity in FSHD, and because of the presence of recombinants in autosomal dominantly inherited families, closer markers or gene definition will be needed for accurate genetic counseling in other situations.