Seventy three sisters of boys with Duchenne muscular dystrophy and their families were studied using up to seven deoxyribonucleic acid (DNA) probes linked to the gene on the short arm of the X chromosome. Fifty three (73%) were informative for flanking markers, a further 18 (24%) being informative for a single marker only. Predictions based on pedigree structure and creatine kinase information from the individuals and their female relatives gave more than half (55%) of the sisters a risk for being a carrier of below 10% or above 90%. The addition of information derived from the inheritance of flanking DNA markers, where they exist, improves the situation so that many more sisters (77%) can be allocated into either high or low risk categories.