A high proportion of females apparently heterozygous for X-linked recessive Duchenne muscular dystrophy were identified by serum enzyme assay in optimal circumstances. A few seemed persistently unidentifiable, and reasons are given that such a limit should exist. Attempts to relate chromosome constitution and phenotype were unsuccessful. Electromyography gave unreliable and often conflicting results.

Five serum enzymes were studied in 27 such female carriers, in several instances serially after standard exercise. The values were highly correlated and declined with age, in contrast to those in normal women; but physical symptoms were associated with elevated values irrespective of age. The diagnostic superiority of creatine phosphokinase was evident throughout. Sibship analyses showed an incidence consistent with complete penetrance of the gene and independent of maternal age. The same enzymes were examined in patients with other muscular dystrophies, and in some illustratory conditions.

Six Duchenne muscular dystrophy pedigrees were ascertained and all the members assessed. The high detection rate disclosed carrier females in otherwise unaffected sibships more than one generation prior to the first afflicted male. Prospective eugenic measures are discussed.