We present the case of a female patient, aged 12 years, with fatigability and exertional myalgias, progressively developed within the last two years. Negative family history, as well as negative personal medical history, were found. At physical examination, short stature, proximal muscle weakness and mild hepatomegaly were noted. Urine ketones level was slightly decreased, serum transaminases, creatine kinase and lactate dehydrogenase levels were increased. Electromyographical examination showed a myopathic non-specific pattern. Deltoid muscle biopsy revealed: small, clear vesicles are present on Hematoxylin-Eosin and modified Gömöri trichrome stains; modified Gömöri trichrome stain also revealed muscle fibers (especially type I of muscle fibers) having mild to moderate mitochondrial proliferation (red rim and speckled sarcoplasm). The lipid storage has been well demonstrated by Sudan Black stain, which revealed small lipid droplets in type I muscle fibers. Abnormal internal architecture with a punctate pattern was showed by adenine dinucleotide tetrazolium reductase and succinate dehydrogenase stains. Electron microscopy showed small inter-myofibrillar accumulations of round, amorphous, homogeneous acellular substances that are not membrane bounded. These features indicate that these are neutral fat (lipid) droplets. Subsarcolemmal accumulations of mitochondria were also revealed. The differential diagnosis of this case is discussed, and the up to date general data concerning carnitine deficiency are presented. The aim of our case-report is to emphasize the role of muscle biopsy in carnitine deficiency, as well as to remind the necessity of keeping in mind such metabolic disorders when doing the differential diagnostic of a muscular weakness.