Applicationochemical techniques to the t study of skeletal muscle biopsies has led to the description of two new clinical pathological entities.1,2The value of these techniques in the study of neuromuscular disease has been detailed and emphasized by Engel.3A third myopathy with histochemically and ultrastructurally abnormal mitochondria has been described.4,5Recently we have obtained muscle biopsies from two patients whose clinical course was characterized by the onset during the first decade of life of progressive muscle fatigability, weakness, and slight atrophy, which was more prominent in the proximal muscles. Only focal fiber generation and occasional subsarcolemmous vacuoles were seen on routine histological sections, but application of histochemical techniques clearly defined abnormal mitochondria. These mitochondria were very active enzymatically and were present in 30% to 40% of the skeletal muscle fibers from both of these patients. Report of Cases Case 1.— A 9-year-old white girl was