A case of a 6-years-old boy with delayed motor milestones, hypotonia since birth (floppy baby), showing a partial improvement in the latter years is reported. On physical examination was found diffuse muscle atrophy, lordosis, generalized hyporeflexia and Gowers maneuver during standing procedure. Serum enzymes were normal and electromyography had potential with increased duration and excess of polyphasic potentials. Fresh-frozen muscle biopsy processed by histochemistry showed type 1 fiber predominance, absence of oxidative enzyme activity in the center of the fibers (central cores) and slight increased of the ATPase reaction in the cores area (structured cores?). Is made a brief discussion about the pathology, pathogenesis, and the good prognosis of the disease.