Internalized capillaries, i.e. capillaries within muscle fibers, represent a rare myopathological feature. This was systematically studied in 923 muscle biopsy specimens and found in 24, chiefly in the gastrocnemius muscle, more rarely in the biceps and quadriceps muscles affecting males more often than females and most frequently associated with juvenile spinal muscular atrophy or Becker's muscular dystrophy. Internalized capillaries, often multiple, ran along the long axis of the muscle fiber within an "internalized" extracellular space and were almost exclusively seen in type I myofibers. Internalization seems to start at the site of fiber splitting while penetration through the intact sarcolemma and invasion into transverse tubules were never observed. The presence of internalized capillaries within type I myofibers and increased density of intramuscular capillaries/fiber though not per muscle fiber area suggested hypoxia to play a possible if not crucial role in the formation of internalized capillaries. Our findings do not distinguish between active proliferation of capillaries into myofibers at the site of myofibers and internalization by fusion of vicinal myofibers as the morphogenetic principles suggest that both of these mechanisms may occur.