Diagnosis of X-linked juvenile retinoschisis (RS) was made in two nonrelated Swedish individuals with restriction fragment length polymorphisms, using probes that flank the RS locus. The X chromosome of a 1-year-old male infant, at risk, displayed the same haplotype as an affected brother for restriction fragment length polymorphisms, representing five linked markers, which extended between the DXS164 and the DXS85 loci and encompassed the RS locus. The diagnosis was confirmed by a clinical examination and an electroretinogram. The X chromosomes of a pregnant woman, an offspring of a carrier female, showed a different haplotype to that of her affected brother, at three linked loci that flanked the RS gene. She was excluded as a carrier with a high probability; hence, the fetus was unlikely to inherit an abnormal gene for this X-linked trait. In informed families a DNA-based diagnosis can serve as a valuable complement to an electroretinogram and a clinical examination in the diagnosis of RS, since these carrier females cannot currently be identified by other means.