Summary:Clinical and electrocardiographic examinations were performed on 29 individuals with X‐linked (Duchenne) pseudohypertrophic muscular dystrophy, three girls and one boy with “Duchennetype” muscular dystrophy and 19 female relatives of the X‐linked Duchenne patients. These female relatives were divided into seven definite, two probable and 10 possible heterozygous carriers of the dystrophic gene. Myocardial dystrophic involvement was manifested by the presence of significant abnormalities in 69% of the electrocardiograms in the Duchenne dystrophic group of patients. There was, however, no correlation between the electrocardiographic appearances and the patient's degree of activity, age or severity of the disease process or the presence of spinal deformities. An abnormal electrocardiogram in this group did not appear to have any prognostic significance.The electrocardiograms in the four children with “Duchenne‐type” muscular dystrophy were normal as were also all but one in the female carrier group, the only significant abnormality being minor non‐specific S‐T and T wave changes in the lateral precordial leads of one recording. None of the carriers demonstrated any muscle weakness. The significance of these findings is discussed especially in regard to potential dystrophic involvement in the heterozygo